Fix some minor mistakes in comments on struct instance

include/pybind11/detail/common.h

@@ -388,7 +388,7 @@ struct instance {
             uint8_t *status;
         } nonsimple;
     };
-    /// Weak references (needed for keep alive):
+    /// Weak references
     PyObject *weakrefs;
     /// If true, the pointer is owned which means we're free to manage it with a holder.
     bool owned : 1;
@@ -405,10 +405,10 @@ struct instance {
      * (which is typically the size of two pointers), or when multiple inheritance is used on the
      * python side.  Non-simple layout allocates the required amount of memory to have multiple
      * bound C++ classes as parents.  Under this layout, `nonsimple.values_and_holders` is set to a
-     * pointer to allocated space of the required space to hold a a sequence of value pointers and
+     * pointer to allocated space of the required space to hold a sequence of value pointers and
      * holders followed `status`, a set of bit flags (1 byte each), i.e.
      * [val1*][holder1][val2*][holder2]...[bb...]  where each [block] is rounded up to a multiple of
-     * `sizeof(void *)`.  `nonsimple.holder_constructed` is, for convenience, a pointer to the
+     * `sizeof(void *)`.  `nonsimple.status` is, for convenience, a pointer to the
      * beginning of the [bb...] block (but not independently allocated).
      *
      * Status bits indicate whether the associated holder is constructed (&