elput: Document how keyboard events are processed

elput: Document how keyboard events are processed

Summary:
Add some light function docs and code comments to explain the steps
followed in processing hardware events for keyboard hits into actual
printable characters.  While this is internal functionality, the process
is important and involves a couple external dependencies (libinput and
libxkbcommon) so documenting this flow may help future developers avoid
introducing bugs.

Signed-off-by: Bryce Harrington <bryce@osg.samsung.com>
Reviewers: zmike

Subscribers: cedric, jpeg

Differential Revision: https://phab.enlightenment.org/D4953