Clarify documentation from `gclient recurse`

Applying a command on a value usually mean to call `cmd value`. Hence
it seemed important to me to clarify that the current working
directory is changed.

Tested:
* Entered `gclient recurse --help` and checked the new message appear.
* Entered both examples in the terminal and checked the output.

Thanks to sdefresne for his help understanding recurse.

Change-Id: I729efc2014ed3cf90112cc89875a283d58ce8af0
Fixed: 1345272
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3769966Reviewed-by: Arthur Milchior <arthurmilchior@google.com>
Reviewed-by: Aravind Vasudevan <aravindvasudev@google.com>
Commit-Queue: Arthur Milchior <arthurmilchior@google.com>

gclient.py

@@ -2251,9 +2251,15 @@ class CipdDependency(Dependency):
 def CMDrecurse(parser, args):
   """Operates [command args ...] on all the dependencies.
 
-  Runs a shell command on all entries.
-  Sets GCLIENT_DEP_PATH environment variable as the dep's relative location to
-  root directory of the checkout.
+  Change directory to each dependency's directory, and call [command
+  args ...] there.  Sets GCLIENT_DEP_PATH environment variable as the
+  dep's relative location to root directory of the checkout.
+
+  Examples:
+  * `gclient recurse --no-progress -j1 sh -c 'echo "$GCLIENT_DEP_PATH"'`
+  print the relative path of each dependency.
+  * `gclient recurse --no-progress -j1 sh -c "pwd"`
+  print the absolute path of each dependency.
   """
   # Stop parsing at the first non-arg so that these go through to the command
   parser.disable_interspersed_args()