input: Document why x-input-send-event is still experimental

The x-input-send-event command was introduced in 2.2 with mention
that it is experimental, but now that several releases have elapsed
without any changes, it would be nice to document why that was done
and should still remain experimental in 2.5.

Meanwhile, our documentation states that we prefer 'lower-case',
rather than 'CamelCase', for qapi enum values.  The InputButton and
InputAxis enums violate this convention.  However, because they are
currently used primarily for generating code that is used internally;
and their only exposure through QMP is via the experimental
'x-input-send-event' command, we are free to change their spelling.
Of course, it would be nicer to delay such a change until the same
time we promote the command to non-experimental.  Adding
documentation will help us remember to do that rename.

We have plans to tighten the qapi generator to flag instances of
inconsistent use of naming conventions; if that lands first, it
will just need to whitelist these exceptions until the time we
settle on the final interface.

Fix a typo in the docs for InputAxis while at it.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1447354243-31825-1-git-send-email-eblake@redhat.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
