add more details for remote mode docs (#366)

examples/trials/mnist-annotation/config_remote.yml

+authorName: default
+experimentName: example_mnist
+trialConcurrency: 1
+maxExecDuration: 1h
+maxTrialNum: 10
+#choice: local, remote, pai
+trainingServicePlatform: remote
+#choice: true, false
+useAnnotation: true
+tuner:
+  #choice: TPE, Random, Anneal, Evolution, BatchTuner
+  #SMAC (SMAC should be installed through nnictl)
+  builtinTunerName: TPE
+  classArgs:
+    #choice: maximize, minimize
+    optimize_mode: maximize
+trial:
+  command: python3 mnist.py
+  codeDir: .
+  gpuNum: 0
+#machineList can be empty if the platform is local
+machineList:
+  - ip: 10.1.1.1
+    username: bob
+    passwd: bob123
+    #port can be skip if using default ssh port 22
+    #port: 22
+  - ip: 10.1.1.2
+    username: bob
+    passwd: bob123
+  - ip: 10.1.1.3
+    username: bob
+    passwd: bob123