diff --git a/bin/named/named.rst b/bin/named/named.rst
index d6a239c282..78af512fff 100644
--- a/bin/named/named.rst
+++ b/bin/named/named.rst
@@ -96,13 +96,17 @@ Options
 
 .. option:: -M option
 
-   This option sets the default memory context options. If set to ``external``,
-   the internal memory manager is bypassed in favor of
-   system-provided memory allocation functions. If set to ``fill``, blocks
-   of memory are filled with tag values when allocated or freed, to
-   assist debugging of memory problems. ``nofill`` disables this behavior,
-   and is the default unless :program:`named` has been compiled with developer
-   options.
+   This option sets the default (comma-separated) memory context
+   options. The possible flags are:
+
+   - ``fill``: fill blocks of memory with tag values when they are
+     allocated or freed, to assist debugging of memory problems; this is
+     the implicit default if :program:`named` has been compiled with
+     ``--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
 
diff --git a/doc/man/named.8in b/doc/man/named.8in
index 2927a6f38a..26722d2c1f 100644
--- a/doc/man/named.8in
+++ b/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,
-the internal memory manager is bypassed in favor of
-system\-provided memory allocation functions. If set to \fBfill\fP, blocks
-of memory are filled with tag values when allocated or freed, to
-assist debugging of memory problems. \fBnofill\fP disables this behavior,
-and is the default unless \fBnamed\fP has been compiled with developer
-options.
+This option sets the default (comma\-separated) memory context
+options. The possible flags are:
+.INDENT 7.0
+.IP \(bu 2
+\fBfill\fP: fill blocks of memory with tag values when they are
+allocated or freed, to assist debugging of memory problems; this is
+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