bug#37016: 26.2; `pushnew' doc string (minor)

[Drew Adams]

1. Why does the first line of the doc string start with the calling
   sequence (it's on the same line as the start of the doc string).

2. The first "sentence" is a bit misleading.  It actually says that X is
   inserted at the head of the list if it is not already at the head of
   the list, which is not right.  It should say something like "if not
   already in the list", or "if not already a member of the list".

3. The doc string doesn't say what PLACE is, except that you can guess
   from the description that it must be a list.  To really understand
   you need to consult the doc of `push'.  The doc of `pushnew' could
   say what is said in the doc of `push', without being longer than it
   is now.

4. Even the doc string of `push' should say that a _list_ is stored in
   PLACE.

   http://clhs.lisp.se/Body/m_push.htm



In GNU Emacs 26.2 (build 1, x86_64-w64-mingw32)
 of 2019-04-13
Repository revision: fd1b34bfba8f3f6298df47c8e10b61530426f749
Windowing system distributor `Microsoft Corp.', version 10.0.17763
Configured using:
 `configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install 'CFLAGS=-O2 -static -g3''