bug#31441: 26; doc of functions `(lax-)plist-get'

[Drew Adams]

> > In addition the manual description of `plist-get' could be improved by
> > mentioning these two things, which are mentioned by its doc string:
> >
> > 1. It uses `eq'.  (If one reads further then this becomes apparent when
> >    reading about `lax-plist-get'.
> 
> The manual says (right above plist-get):
> 
> They all compare property names using @code{eq}.

... which is belied further down by the description
of `lax-plist-get'.

That's the point.  The intro says that they all use
`eq' - which is wrong.  The entry for `plist-get'
says nothing about comparison.  The entry for `lax...'
says it uses `equal'.

Parallel construction calls for each to say what it
uses, and remove the statement saying that they all
use `eq'.

This is about making the doc clearer.