gclient template file clarifications (#160882)

A few clarifications in the gclient file templates:

* Sort sections such that RBE and standard variants are consistent.
* Add web engine instructions in comments.
* Explain how to use SSH-based GitHub checkouts.

Author: yjbanov

engine/scripts/rbe.gclient

@@ -2,13 +2,20 @@
 # or just run gclient sync in an empty directory with this file.
 solutions = [
   {
-    "custom_vars": {
-      "use_rbe": True,
-    },
     "deps_file": "DEPS",
     "managed": False,
     "name": ".",
     "safesync_url": "",
+
+    # If you are using SSH to connect to GitHub, change the URL to:
+    # git@github.com:flutter/flutter.git
     "url": "https://github.com/flutter/flutter.git",
+
+    "custom_vars": {
+      "use_rbe": True,
+
+      # Uncomment download_emsdk below if you plan to build the web engine.
+      # "download_emsdk": True,
+    },
   },
 ]

engine/scripts/standard.gclient

@@ -7,6 +7,14 @@ solutions = [
     "managed": False,
     "name": ".",
     "safesync_url": "",
+
+    # If you are using SSH to connect to GitHub, change the URL to:
+    # git@github.com:flutter/flutter.git
     "url": "https://github.com/flutter/flutter.git",
+
+    # Uncomment the custom_vars section below if you plan to build the web engine.
+    # "custom_vars": {
+    #   "download_emsdk": True,
+    # },
   },
 ]