Update submodule_update comment to new functionality.

submodule_update can now return the command it ultimately executes so
the comment above it reflects that.

It now expects a string as an optional argument.

Change trailing space above to period for correct grammar.
Merge update-submodules-docs (#94) into master

Update submodule_update comment for new functionality.

submodule_update can now return the command it ultimately executes so
the comment above it reflects that.

It now expects a string as an optional argument.

Change trailing space above to period for correct grammar.