doc: clarify that df now generally processes special files correctly

doc: clarify that df now generally processes special files correctly

Since v8.21-172-g33660b4, df not only treats symbolic link arguments
differently, as stated there, but now generally processes special file
arguments in a non-canonicalized form correctly:

  $ cd /dev && df-old sdb
  Filesystem     1K-blocks  Used Available Use% Mounted on
  devtmpfs         1014572    48   1014524   1% /dev

  $ cd /dev && df-new sdb
  Filesystem     1K-blocks    Used Available Use% Mounted on
  /dev/sdb        10190136 6039532   3609932  63% /home

Document df's new behavior.

* doc/coreutils.texi (df invocation): In the paragraph describing
df's behavior regarding special file arguments, relax the condition
for such special files from "... is an absolute name of ..." to
"... resolves to ...".
* NEWS (Bug fixes): Mention the new behavior also here.