doc: prepare for a world without whatchanged

Some documentation examples reference "whatchanged", either as a
placeholder command or an example of source structure.

To reduce the need for future edits when `whatchanged` is removed,
replace these references with alternatives:

 - In `MyFirstObjectWalk.adoc`, use `version` as the nearby anchor
   point for `walken`, instead of `whatchanged`.

 - In `user-manual.adoc`, cite `show` instead of `whatchanged` as
   a command whose source lives in the same file as `log`.

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

Documentation/MyFirstObjectWalk.adoc

@@ -83,13 +83,13 @@ int cmd_walken(int argc, const char **argv, const char *prefix)
 }
 ----
 
-Also add the relevant line in `builtin.h` near `cmd_whatchanged()`:
+Also add the relevant line in `builtin.h` near `cmd_version()`:
 
 ----
 int cmd_walken(int argc, const char **argv, const char *prefix);
 ----
 
-Include the command in `git.c` in `commands[]` near the entry for `whatchanged`,
+Include the command in `git.c` in `commands[]` near the entry for `version`,
 maintaining alphabetical ordering:
 
 ----

Documentation/user-manual.adoc

@@ -4240,7 +4240,7 @@ command `git`.  The source side of a builtin is
 - an entry in `BUILTIN_OBJECTS` in the `Makefile`.
 
 Sometimes, more than one builtin is contained in one source file.  For
-example, `cmd_whatchanged()` and `cmd_log()` both reside in `builtin/log.c`,
+example, `cmd_show()` and `cmd_log()` both reside in `builtin/log.c`,
 since they share quite a bit of code.  In that case, the commands which are
 _not_ named like the `.c` file in which they live have to be listed in
 `BUILT_INS` in the `Makefile`.