bug#13371: removing @acronym from manual

[Karl Berry]

    the latest Texinfo documentation (from the 4.13.93 pretest) says
    nothing about that

On the contrary, I added (brief) text to both the acronym and Smallcaps
nodes giving caveats about using @acronym and @sc quite a while ago.
It's been in all the pretests.  I've retweaked that text again just now,
though, so I'm glad you mentioned it.

Anyway, avoiding them is not official requirement, not at all.  Not even
an official recommendation.  I just came to the conclusion when people
asked my advice some years ago -- I observed that whenever a manual
tries to use @acronym or @sc for normal all-caps abbreviations, such as
"GNU", in practice the commands are not used consistently (as we have
just seen in the coreutils case).

Since there's no special benefit to using @acronym{GNU} or @sc{gnu} over
just plain "GNU", it's merely about typographic preferences, my
suggestion was that it was better to avoid the whole issue, simplify the
source, and go with "GNU".

Nevertheless, if a GNU manual wants to use them, that is fine by me.

k