Improve documentation around HostnameVerifier

Discourage use of HttpsURLConnect.setDefaultHostnameVerifier()
and SSLContext.setDefault(). Clarify behavior of HostnameVerifier.

Bug: 126851476
Test: make docs / inspection
Merged-In: Ief47cce8cc71b996491706ea723ef9c4cdd90ca6
Change-Id: Ief47cce8cc71b996491706ea723ef9c4cdd90ca6
(cherry picked from commit cc7958f9)