flock: Update documentation

[Bruno Haible]

The documentation of the Gnulib module 'flock' says that no replacement is
provided on AIX, HP-UX, Solaris. This is wrong: The lib/flock.c file _does_
provide a replacement in this case, based on fcntl. However, I see a unit
test failure for it on Solaris 11.3 and AIX 7.1.

I don't know if this is due to different semantics of locking facilities,
as described in
http://www.perkin.org.uk/posts/solaris-portability-flock.html
https://gavv.github.io/articles/file-locks/
https://unix.stackexchange.com/questions/1777/flock2-versus-fcntl2-over-a-nfs


2021-01-06  Bruno Haible  <bruno@clisp.org>

        flock: Update documentation.
        * doc/glibc-functions/flock.texi: Document what to expect on AIX and
        Solaris.

diff --git a/doc/glibc-functions/flock.texi b/doc/glibc-functions/flock.texi
index 41cca4c..bc93a67 100644
--- a/doc/glibc-functions/flock.texi
+++ b/doc/glibc-functions/flock.texi
@@ -13,11 +13,12 @@ Portability problems fixed by Gnulib:
 @item
 This function is missing on some platforms:
 mingw, MSVC 14
+@item
+This function is missing on some platforms:
+AIX 7.1, HP-UX 11.23, Solaris 11.4.
+But the replacement function does not really work; it exhibits test failures.
 @end itemize
 
 Portability problems not fixed by Gnulib:
 @itemize
-@item
-This function is missing on some platforms:
-AIX 5.1, HP-UX 11.23, Solaris 11.4.
 @end itemize