RFR: Comment detailing refspec format in GitRepository#push

[Julian Waters]

Small change to document the git refspec field in GitRepository#push in a comment

-------------

Commit messages:
 - Documentation

Changes: https://git.openjdk.org/skara/pull/1448/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1448&range=00
  Stats: 7 lines in 1 file changed: 7 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/skara/pull/1448.diff
  Fetch: git fetch https://git.openjdk.org/skara pull/1448/head:pull/1448

PR: https://git.openjdk.org/skara/pull/1448