Update PJSUA2 docs on authentication digest AKA callback

[nanangizz]

Re #4084, the docs of PJSUA2 Endpoint::onCredAuth() says:

If PJSIP_HAS_DIGEST_AKA_AUTH is disabled, you have to overload
this callback to provide your own digest computation back-end.

while the digest AKA authentication requires PJSIP_HAS_DIGEST_AKA_AUTH to be set/enabled.

This PR fixes Endpoint::onCredAuth() docs and few minors (e.g: indentations, column width limit), no code/logic is changed.