diff --git a/doc/autoconf.texi b/doc/autoconf.texi
index 43a84bc..31a55c0 100644
--- a/doc/autoconf.texi
+++ b/doc/autoconf.texi
@@ -3082,10 +3082,9 @@ AC_CHECK_HEADERS([unistd.h])
 @end example
 
 @noindent
-Then you could have code like the following in @file{conf.h.in}.  On
-systems that have @file{unistd.h}, @command{configure} defines
address@hidden to 1.  On other systems, the whole line is
-commented out (in case the system predefines that symbol).
+Then you could have code like the following in @file{conf.h.in}.
+The @file{conf.h} created by @command{configure} defines @samp{HAVE_UNISTD_H}
+to 1, if and only if the system has @file{unistd.h}.
 
 @example
 @group
@@ -3095,28 +3094,40 @@ commented out (in case the system predefines that symbol).
 @end example
 
 Pay attention that @samp{#undef} is in the first column, and there is
-nothing after @samp{HAVE_UNISTD_H}, not even white space.  You can
-then decode the configuration header using the preprocessor directives:
+nothing after @samp{HAVE_UNISTD_H}, not even white space.
+(The use of old form templates, with @samp{#define} instead of @samp{#undef}
+or comments on the same line as the @samp{#undef}, is strongly discouraged.)
 
+Since it is a tedious task to keep a template header up to date, you may
+use @command{autoheader} to generate it, see @ref{autoheader Invocation}.
+
+During the instantiation of the header, each @samp{#undef} line in the
+template file for each symbol defined by @samp{AC_DEFINE} is changed to an
+appropriate @samp{#define}. If the corresponding @samp{AC_DEFINE} has not
+been executed during the @command{configure} run, the @samp{#undef} line is
+commented out.  (This is important, e.g., for @samp{_POSIX_SOURCE}:
+on many systems, it shall not be undefined nor redefined, yet
address@hidden has to define it on Minix.)
+
+Currently, @emph{all} remaining @samp{#undef} lines in the header
+template are commented out; but this behavior is not guaranteed for future
+releases of Autoconf.
+
+Generally speaking, all @samp{#define} and @samp{#undef} directives in the
+header template might be replaced, under some circumstances.
+Consequently, if you need to use the construct
 @example
 @group
-#include <conf.h>
-
-#ifdef HAVE_UNISTD_H
-# include <unistd.h>
-#else
-/* We are in trouble.  */
-#endif
+#ifdef THIS
+# define THAT
+...
 @end group
 @end example
-
-The use of old form templates, with @samp{#define} instead of
address@hidden is strongly discouraged.  Similarly with old templates
-with comments on the same line as the @samp{#undef}.  Anyway, putting
-comments in preprocessor macros has never been a good idea.
-
-Since it is a tedious task to keep a template header up to date, you may
-use @command{autoheader} to generate it, see @ref{autoheader Invocation}.
+you should better place it outside of the template.
+If you absolutely need to hook it to the config header itself, please put
+the directives to a separate file, and @samp{#include} that file from the
+config header template.  If you are using @command{autoheader}, you would
+probably use @samp{AH_BOTTOM} to append the @samp{#include} directive.
 
 
 @node autoheader Invocation