[docs][R-package] Update the explanation of num_threads (fixes #4192) (#4199)

* update the explanation of num_threads in lightgbm.R

* adjusted sentences for linting.

* remove white space for lintr pass

* add note about num_thread param in files

R-package/R/lgb.cv.R

@@ -46,8 +46,9 @@ CVBooster <- R6::R6Class(
 #'                \item{\code{max_depth}: Limit the max depth for tree model. This is used to deal with
 #'                                 overfit when #data is small. Tree still grow by leaf-wise.}
 #'                \item{\code{num_threads}: Number of threads for LightGBM. For the best speed, set this to
-#'                                   the number of real CPU cores, not the number of threads (most
-#'                                   CPU using hyper-threading to generate 2 threads per CPU core).}
+#'                             the number of real CPU cores(\code{parallel::detectCores(logical = FALSE)}),
+#'                             not the number of threads (most CPU using hyper-threading to generate 2 threads
+#'                             per CPU core).}
 #'            }
 #' @inheritSection lgb_shared_params Early Stopping
 #' @return a trained model \code{lgb.CVBooster}.

R-package/R/lgb.train.R

@@ -19,8 +19,9 @@
 #'                \item{\code{max_depth}: Limit the max depth for tree model. This is used to deal with
 #'                                 overfit when #data is small. Tree still grow by leaf-wise.}
 #'                \item{\code{num_threads}: Number of threads for LightGBM. For the best speed, set this to
-#'                                   the number of real CPU cores, not the number of threads (most
-#'                                   CPU using hyper-threading to generate 2 threads per CPU core).}
+#'                             the number of real CPU cores(\code{parallel::detectCores(logical = FALSE)}),
+#'                             not the number of threads (most CPU using hyper-threading to generate 2 threads
+#'                             per CPU core).}
 #'            }
 #' @inheritSection lgb_shared_params Early Stopping
 #' @return a trained booster model \code{lgb.Booster}.

R-package/R/lightgbm.R

@@ -93,8 +93,9 @@ NULL
 #'         \item{\code{max_depth}: Limit the max depth for tree model. This is used to deal with
 #'                          overfit when #data is small. Tree still grow by leaf-wise.}
 #'          \item{\code{num_threads}: Number of threads for LightGBM. For the best speed, set this to
-#'                             the number of real CPU cores, not the number of threads (most
-#'                             CPU using hyper-threading to generate 2 threads per CPU core).}
+#'                             the number of real CPU cores(\code{parallel::detectCores(logical = FALSE)}),
+#'                             not the number of threads (most CPU using hyper-threading to generate 2 threads
+#'                             per CPU core).}
 #'     }
 #' @inheritSection lgb_shared_params Early Stopping
 #' @return a trained \code{lgb.Booster}

R-package/man/lgb.cv.Rd

@@ -121,8 +121,9 @@ into a predictor model which frees up memory and the original datasets}
     \item{\code{max_depth}: Limit the max depth for tree model. This is used to deal with
                      overfit when #data is small. Tree still grow by leaf-wise.}
     \item{\code{num_threads}: Number of threads for LightGBM. For the best speed, set this to
-                       the number of real CPU cores, not the number of threads (most
-                       CPU using hyper-threading to generate 2 threads per CPU core).}
+                 the number of real CPU cores(\code{parallel::detectCores(logical = FALSE)}),
+                 not the number of threads (most CPU using hyper-threading to generate 2 threads
+                 per CPU core).}
 }}
 }
 \value{

R-package/man/lgb.train.Rd

@@ -104,8 +104,9 @@ original datasets}
     \item{\code{max_depth}: Limit the max depth for tree model. This is used to deal with
                      overfit when #data is small. Tree still grow by leaf-wise.}
     \item{\code{num_threads}: Number of threads for LightGBM. For the best speed, set this to
-                       the number of real CPU cores, not the number of threads (most
-                       CPU using hyper-threading to generate 2 threads per CPU core).}
+                 the number of real CPU cores(\code{parallel::detectCores(logical = FALSE)}),
+                 not the number of threads (most CPU using hyper-threading to generate 2 threads
+                 per CPU core).}
 }}
 }
 \value{

R-package/man/lightgbm.Rd

@@ -66,8 +66,9 @@ If early stopping occurs, the model will have 'best_iter' field.}
     \item{\code{max_depth}: Limit the max depth for tree model. This is used to deal with
                      overfit when #data is small. Tree still grow by leaf-wise.}
      \item{\code{num_threads}: Number of threads for LightGBM. For the best speed, set this to
-                        the number of real CPU cores, not the number of threads (most
-                        CPU using hyper-threading to generate 2 threads per CPU core).}
+                        the number of real CPU cores(\code{parallel::detectCores(logical = FALSE)}),
+                        not the number of threads (most CPU using hyper-threading to generate 2 threads
+                        per CPU core).}
 }}
 }
 \value{