Update documentation for changed splitGitDiffPaths function
The behavior (and name) of this function was changed in
D16405, but the documentation was not updated to reflect the new
Test Plan: Untested; pure doc changes.
Reviewers: Blessed Reviewers, epriestley
Reviewed By: Blessed Reviewers, epriestley
Differential Revision: https://secure.phabricator.com/D16425