Clarify docs on GN rebase_path.

A question came up in a code review, so this adds more explicit documentation
and hints for paths ending (or not) in slashes.

Review URL: https://codereview.chromium.org/1563543002

Cr-Original-Commit-Position: refs/heads/master@{#367623}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: e3b32676059d25162cd07201bbd33dfec3c60daf
1 file changed
tree: 04c5c35ddbbc26257bcaa39873464a7da38ceb4f
  1. tools/