autoconf: doc: document BusyBox sed w bug

From d405fa2ec63b0e4ebb9f84d1a4b2a6de63b2d26d Mon Sep 17 00:00:00 2001
From: Paul Eggert <[EMAIL REDACTED]>
Date: Sat, 1 Jan 2022 18:08:15 -0800
Subject: [PATCH] doc: document BusyBox sed w bug

 doc/autoconf.texi | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/doc/autoconf.texi b/doc/autoconf.texi
index 17a6326e..04730dcc 100644
--- a/doc/autoconf.texi
+++ b/doc/autoconf.texi
@@ -20170,6 +20170,30 @@ s/.*/deleted/g
 @end example
+@item @command{sed} (@samp{w})
+@c ---------------------------
+@prindex @command{sed} (@samp{w})
+When a script contains multiple commands to write lines to the same
+output file, BusyBox @command{sed} mistakenly opens a separate output
+stream for each command.  This can cause one of the commands to ``win''
+and the others to ``lose'', in the sense that their output is discarded.
+For example:
+sed -n -e '
+  /a/w xxx
+  /b/w xxx
+' <<EOF
+@end example
+This might output only @samp{a} to @file{xxx}; the @samp{b} is lost.
+To avoid the problem, a portable script should contain at most one
+@samp{w} or @samp{s/.../.../w} command per output file.
 @item @command{sleep}
 @c ------------------
 @prindex @command{sleep}