doc: prepare for a world without whatchanged

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>