bug#25462: Out of date doc string for "`".

[Stefan Kangas]

Alan Mackenzie <acm@muc.de> writes:

> Hello, Emacs.
>
> The doc string for "`" has become partial.
>
> It fails to mention that "`" has a radically different meaning when used
> in certain parts of a `pcase', and probably quite a few other similar
> macros.

How about adding this at the very end of the ` docstring:

    Note that some macros, such as `pcase', change the meaning of this
    symbol.