doc bug: update-ref --create-reflog also works with --stdin

This is to report a bug in the docs, or at least suggest an improvement.

The update-ref documentation says that --create-reflog can only be passed when providing a new ref to create and doesn't say it can also be used with --stdin:

'git update-ref' [-m <reason>] [--no-deref] (-d <ref> [<oldvalue>] | [--create-reflog] <ref> <newvalue> [<oldvalue>] | --stdin [-z])

from: https://github.com/git/git/blob/ffac537e6cbbf934b08745a378932722df287a53/Documentation/git-update-ref.txt#L11

My testing shows that it also works with --stdin, so it seems like it would help to document this.

Thanks, --Chris