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.

Bug: 80238372
Test: make ds-docs with visual inspection of generated HTML
Change-Id: I460d70d692cc1a04cf0efd6b9d0b1e13a9403968
Merged-In: Id4a00c1a44dc3cbbaefc94c0438eb1a077f06e5e