Add hint regarding possible constraints for auth-id values.
Add hint regarding possible constraints for auth-id values.

Certain authentication mechanisms may restrict the characters being
usable for the authentication identifier. The Credentials Format section
now contains a corresponding hint.

Fixes #430.

Signed-off-by: Kai Hudalla <6c3288b94355003a6c5f12ba233f9fc06ef02a62@bosch-si.com>
dejanb/hono,dejanb/hono,dejanb/hono