Spring ACLs documentation on accessing principle confusing

[jroper]

The documentation on accessing principals in ACLs is confusing. There are two separate sections on it, one that says you can access it through the _kalix-src-svc and _kalix-src header, and the other that says you can access it through Metadata.principals(). It's not clear at all that these are referring to the same thing, moreover, the headers we shouldn't really be documenting at all. We should just describe how to access the principal using the Metadata.principals() call. All _kalix-* prefixed headers should be considered internal implementation details, there should always be a corresponding API to work with them, they should never need to be documented directly.

[octonato]

For the record, the Protobuf docs as well. https://docs.kalix.io/java-protobuf/access-control.html#_programmatically_accessing_principals