bug#47051: 27.1; Incorrect backward-button signature in elisp manual

[Howard Melman]

In the Emacs 27.1 lispref manual node Button Buffer Commands:

The documentation of backward-button mentions the last
argument NO-ERROR but doesn't include it on the command
signature.

Also the last sentence of the descriptions for both
backward-button and forward-button says:

    If NO-ERROR in non-‘nil’

and I think both should be:

    If NO-ERROR is non-‘nil’

I believe these are still problems in master.

The docstrings of both commands seem fine.

-- 

Howard