whatschanged: list it in BreakingChanges document

This can be squashed into the previous step.  That is how our "git
pack-redundant" conversion did.

Theoretically, however, those who want to gauge the need to keep the
command by exposing their users to patches before this one may want
to wait until their experiment finishes before they formally say
"this will go away".

This change is made into a separate patch from the previous step
precisely to help those folks.

While at it, update the documentation page to use the new [synopsis]
facility to mark-up the SYNOPSIS part.

Helped-by: Elijah Newren <newren@gmail.com>
[en: typofix]
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/BreakingChanges.adoc

@@ -178,6 +178,15 @@ references.
 +
 These features will be removed.
 
+* The git-whatchanged(1) command has outlived its usefulness more than
+  10 years ago, and takes more keystrokes to type than its rough
+  equivalent `git log --raw`.  We have nominated the command for
+  removal, have changed the command to refuse to work unless the
+  `--i-still-use-this` option is given, and asked the users to report
+  when they do so.  So far there hasn't been a single complaint.
++
+The command will be removed.
+
 == Superseded features that will not be deprecated
 
 Some features have gained newer replacements that aim to improve the design in

Documentation/git-whatchanged.adoc

@@ -8,8 +8,14 @@ git-whatchanged - Show logs with differences each commit introduces
 
 SYNOPSIS
 --------
-[verse]
-'git whatchanged' <option>...
+[synopsis]
+git whatchanged <option>...
+
+WARNING
+-------
+`git whatchanged` has been deprecated and is scheduled for removal in
+a future version of Git, as it is merely `git log` with different
+default; `whatchanged` is not even shorter to type than `log --raw`.
 
 DESCRIPTION
 -----------