Update documentation for changed splitGitDiffPaths function

Summary:
The behavior (and name) of this function was changed in
D16405, but the documentation was not updated to reflect the new
contract.

Test Plan: Untested; pure doc changes.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D16425