Document 'enriched-toggle-markup'

* etc/enriched.txt: Document the new 'enriched-toggle-markup'
command.  (Bug#33855)

etc/NEWS

@@ -822,7 +822,7 @@ so automatically.
 
 ** Enriched Mode
 
----
++++
 *** New command 'enriched-toggle-markup'.
 This allows you to see the markup in 'enriched-mode' buffers (e.g.,
 the HELLO file).

etc/enriched.txt

@@ -137,7 +137,7 @@ Unfortunately this does not look as nice with a fixed-width font
 as it does in a proportionally-spaced printed document; the extra
 spaces that are needed on the screen can make it hard to read.</flushboth>
 
-<center>
+    <center>
 
     <bold>Center</bold>
 
@@ -149,7 +149,6 @@ spaces that are needed on the screen can make it hard to read.</flushboth>
     justification.
 
 
-
 </center><flushboth>Note that justification can only change at hard newlines, because
 that is the unit over which filling gets done.</flushboth></indent>
 
@@ -189,7 +188,14 @@ as possible.
 
 
 The text/enriched standard is defined in </indent>Internet<indent> RFC 1896
-(<<http://www.ietf.org/rfc/rfc1896.txt>).</indent>
+(<<http://www.ietf.org/rfc/rfc1896.txt>).
+
+
+To make format annotations visible and possibly edit them, tell
+Emacs to display the markup:
+
+
+<fixed><indent>M-x enriched-toggle-markup RET</indent></fixed></indent>
 
 
 <x-bg-color><param>blue</param><x-color><param>white</param><bold>CUSTOMIZATION</bold></x-color></x-bg-color><bold>