bug#17007: 24.3.50; describe-key/function evaluates documentation function in the wrong buffer

[martin rudalics]

> This has stopped working in 24.3.5 since `with-help-window' started
> replaced `with-output-to-temp-buffer' with
> `with-temp-buffer-window'. The former just binds `standard-output' while
> the latter also sets the current buffer to the *Help* buffer.
>
> The result is that the fallback keybinding reported is always
> "forward-button", which is almost always wrong.
>
> One could either
>
> 1. revert that change (was it just a cleanup?)
> 2. fix/parametrize that particular behaviour of
> `with-temp-buffer-window'
> 3. pass an extra original-buffer arg to `describe-function-1'
> 4. dynamically bind some new `help-original-buffer' var.
>
> Even though a better mechanism for "fallback keybindings" is being
> discussed (and by then yasnippet can get rid of its own technique, which
> is half-baked but working since emacs 22), it'd be nice if the current
> function-documentation trick is kept working for upcoming emacs 24.4.
>
> This originated in https://github.com/capitaomorte/yasnippet/issues/468
>
> http://github.com/capitaomorte/autopair does something similar and is
> probably also affected, but I plan to deprecate autopair in favor of
> 24.4's electric-pair-mode anyway.

Sorry, I messed this up too often already.  Hopefully, it's sufficient
to add one `with-current-buffer' binding at some particular level.
Could you please send me the calling sequence used by yasnippet, so I
can identify the location where this is necessary?

martin