add a few more docs to existing funcs

Change-Id: I27317a59aba67c05ca1fd333e8f064c0edccb209
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/343185
Reviewed-by: LaMont Jones <lamontjones@google.com>
Tested-by: Mike Frysinger <vapier@google.com>

git_command.py

@@ -158,6 +158,8 @@ def git_require(min_version, fail=False, msg=''):
 
 
 class GitCommand(object):
+  """Wrapper around a single git invocation."""
+
   def __init__(self,
                project,
                cmdv,