Add a note to Basic Auth Section mentioning that it will only work with resident KM

[AnuGayan]

Current Limitation

If third-party KM configured , then you Basic Auth with the IS provisioned users. We only support OAuth in that use case. Even though we haven't explicitly said that in the docs, we have mentioned that token validation is for JWT in several places.
This need to be properly added to the doc.

Suggested Improvement

Improve the Basic auth section mentioning above

Version

4.6.0