docs: mention that the 'formatter' command has been superseded

doc/nano.texi

@@ -1035,6 +1035,22 @@ in one of the system-installed files (which normally are not writable).
 
 @end table
 
+Note: the @code{formatter} command has been removed.  It was superseded by
+a more general mechanism: the filtering of buffer or marked text through
+an external command.  Such filtering is done by typing @code{^R^X} and then
+preceding your formatter command with the pipe symbol (@code{|}).  It has
+the added advantage that the operation can be undone.
+
+If you use such a formatting command regularly, you could assign the relevant
+series of keystrokes to a single key in your nanorc:
+
+@example
+   bind  M-F  "^R^X|yourformatcommand^M"  main
+@end example
+
+(Note that the @key{^R}, @key{^X}, and @key{^M} are each a single, literal
+control character.  You can enter them by preceding each with @key{M-V}.)
+
 @node Rebinding Keys
 @section Rebinding Keys
 

doc/nanorc.5

@@ -391,6 +391,23 @@ command to an already defined syntax -- useful when you want to
 slightly improve a syntax defined in one of the system-installed
 files (which normally are not writable).
 
+.P
+Note: the \fBformatter\fR command has been removed.  It was superseded by
+a more general mechanism: the filtering of buffer or marked text through
+an external command.  Such filtering is done by typing \fB^R^X\fR and then
+preceding your formatter command with the pipe symbol (\fB|\fR).  It has
+the added advantage that the operation can be undone.
+.PP
+If you use such a formatting command regularly, you could assign the relevant
+series of keystrokes to a single key in your nanorc:
+.PP
+.RS
+.B "bind  M\-F  \(dq^R^X|yourformatcommand^M\(dq  main"
+.RE
+.PP
+(Note that the \fB^R\fR, \fB^X\fR, and \fB^M\fR are each a single, literal
+control character.  You can enter them by preceding each with \fBM-V\fR.)
+
 .SH REBINDING KEYS
 Key bindings can be changed via the following three commands:
 .RS 3