update explicit-cache with note saying attributes are now supported

[alexcb]

Does existing documentation exist?
https://docs.earthly.dev/docs/caching/caching-via-registry#explicit-cache-advanced

Request
#3714 and #3868 are now fixed; however we should document that this new feature exists.

For example, users can now run:

earthly --remote-cache=harbor.company.team/earthly-cache/test:cache,image-manifest=true,oci-mediatypes=true +example

for cases where their registry requires additional attributes to be set.

[idodod]

addressed in #3920