Docs: noted that setting proxy hostnames on an ApnSetting requires an internet connection for DNS.

Doc-only change.
Also improved the class descriptions and fixed some typos.

Staged at:
  http://go/dac-stage/reference/android/telephony/data/ApnSetting_new
  http://go/dac-stage/reference/android/telephony/data/ApnSetting.Builder_new

Bug: 80238372
Test: make ds-docs with visual inspection of generated HTML
Change-Id: Id4a00c1a44dc3cbbaefc94c0438eb1a077f06e5e