Improve docs about //visibility:public

https://r.android.com/3252814 modified the visibility to
//visibility:public to account for visibility skew between development
and release branches.

Drop the comment about stub libraries being safe to use by any _rdep_.
There might be cases where this is not true.

Test: N/A
Bug: 239189932
Change-Id: I74ca661a40384d480148f5e18f76495e837c58e3