Update documentation for named's -M option

Remove "external" from the list of legal values for the -M command-line
option as it has not been allowed since the internal memory allocator
was removed by commit 55ace5d3aa.

Make the style of the relevant paragraph more in line with the next one
and split its contents up into an unordered list of options for improved
readability.

bin/named/named.rst

@@ -96,13 +96,17 @@ Options
 
 .. option:: -M option
 
-   This option sets the default memory context options. If set to ``external``,
+   This option sets the default (comma-separated) memory context
-   the internal memory manager is bypassed in favor of
+   options. The possible flags are:
-   system-provided memory allocation functions. If set to ``fill``, blocks
+
-   of memory are filled with tag values when allocated or freed, to
+   - ``fill``: fill blocks of memory with tag values when they are
-   assist debugging of memory problems. ``nofill`` disables this behavior,
+     allocated or freed, to assist debugging of memory problems; this is
-   and is the default unless :program:`named` has been compiled with developer
+     the implicit default if :program:`named` has been compiled with
-   options.
+     ``--enable-developer``.
+
+   - ``nofill``: disable the behavior enabled by ``fill``; this is the
+     implicit default unless :program:`named` has been compiled with
+     ``--enable-developer``.
 
 .. option:: -m flag
 

doc/man/named.8in

@@ -113,13 +113,19 @@ This option sets the log to the file \fBlogfile\fP by default, instead of the sy
 .INDENT 0.0
 .TP
 .B \-M option
-This option sets the default memory context options. If set to \fBexternal\fP,
+This option sets the default (comma\-separated) memory context
-the internal memory manager is bypassed in favor of
+options. The possible flags are:
-system\-provided memory allocation functions. If set to \fBfill\fP, blocks
+.INDENT 7.0
-of memory are filled with tag values when allocated or freed, to
+.IP \(bu 2
-assist debugging of memory problems. \fBnofill\fP disables this behavior,
+\fBfill\fP: fill blocks of memory with tag values when they are
-and is the default unless \fBnamed\fP has been compiled with developer
+allocated or freed, to assist debugging of memory problems; this is
-options.
+the implicit default if \fBnamed\fP has been compiled with
+\fB\-\-enable\-developer\fP\&.
+.IP \(bu 2
+\fBnofill\fP: disable the behavior enabled by \fBfill\fP; this is the
+implicit default unless \fBnamed\fP has been compiled with
+\fB\-\-enable\-developer\fP\&.
+.UNINDENT
 .UNINDENT
 .INDENT 0.0
 .TP