open_memstream: update docs wrt posix issue 8

[DJ Delorie]

Issue 8 adds this text:

  "The fseek() and fseeko() functions can be used to set the file
   position beyond the current buffer length. It is
   implementation-defined whether this extends the buffer to the new
   length. If it extends the buffer, the added buffer contents shall
   be set to null bytes for open_memstream(), or null wide characters
   for open_wmemstream(); if it does not extend the buffer, then if
   data is later written at this point, the buffer contents in the gap
   shall be set to null bytes for open_memstream(), or null wide
   characters for open_wmemstream(). If fseek() or fseeko() is called
   with SEEK_END as the whence argument, it is implementation-defined
   whether the file position is adjusted relative to the current
   buffer length or relative to the buffer size that would be set by
   an fflush() call made immediately before the fseek() or fseeko()
   call."

https://pubs.opengroup.org/onlinepubs/9799919799/functions/open_memstream.html
https://www.austingroupbugs.net/view.php?id=1406&nbn=6

To define our implementation, I propose the following, which covers
both "it is implementation-defined whether the file position is
adjusted relative to the current buffer length or relative to the
buffer size" as well as addressing the original bug report that
triggered this path:

--- patch follows ---

manual: Explain our implementation-defined memstream semantics

Posix Issue 8 adds an implementation-defined item we don't already
cover, about seeking backwards.  This defines our implentation.

diff --git a/manual/stdio.texi b/manual/stdio.texi
index 16d459424e..3ae906cdee 100644
--- a/manual/stdio.texi
+++ b/manual/stdio.texi
@@ -5141,7 +5141,12 @@ is @emph{not} included in the size value stored at @var{sizeloc}.
 You can move the stream's file position with @code{fseek} or
 @code{fseeko} (@pxref{File Positioning}).  Moving the file position past
 the end of the data already written fills the intervening space with
-zeroes.
+zeroes.  Note that seeking backwards into existing written data will
+change the effective ``end of file'' used by @code{fflush} and
+@code{SEEK_END}.  Thus, if you wish to do ``random-access'' I/O in a
+memstream, it's important to use @code{fseek} to move the file
+position to the desired data end (using @code{SEEK_POS}) before
+flushing or closing it.
 @end deftypefun
 
 Here is an example of using @code{open_memstream}: