autoconf: doc: Document how to deal with missing 'join'.

From b0d5d2d3eb05cf765c44f0499a622a99319d629f Mon Sep 17 00:00:00 2001
From: Bruno Haible <[EMAIL REDACTED]>
Date: Sat, 8 Jan 2022 22:31:58 +0100
Subject: [PATCH] doc: Document how to deal with missing 'join'.

* doc/autoconf.texi (Limitations of Usual Tools): Mention that 'join' is missing
in BusyBox.
 doc/autoconf.texi | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/doc/autoconf.texi b/doc/autoconf.texi
index 04730dcc..06a98dd0 100644
--- a/doc/autoconf.texi
+++ b/doc/autoconf.texi
@@ -19624,6 +19624,12 @@ On NetBSD, @command{join -a 1 file1 file2} mistakenly behaves like
 @command{join -a 1 -a 2 1 file1 file2}, resulting in a usage warning;
 the workaround is to use @command{join -a1 file1 file2} instead.
+On platforms with the BusyBox tools, the @command{join} command is
+entirely missing.  As a workaround, you can simulate special cases of the
+@command{join} command using an @command{awk} script.  For an example,
+see @url{}.
 @item @command{ln}
 @c ---------------
 @prindex @command{ln}