Docs: tell maintainers to put [GIT PULL] in their subject lines

It seems that Linus looks for [GIT PULL] in subject lines to ensure that
pull requests don't get buried in the noise during merge windows.  Update
the docs to reflect that.

[jc: From an impromptu post from willy, thus no SOB]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>