Work for #1866
Migrates all the public class docs over to appledoc format. While Xcode is fine with blank lines in `///` comments, appledoc (used by cocoadocs) isn't and was leaving a bunch of info off the doc pages.
The generator still needs to be updated to do this also; that will be a follow up CL.
- Convert most of the core library headers over to HeaderDoc format.
- Switch the generated comments over to HeaderDoc.
- Create GPBCodedOutputStream_PackagePrivate and move some things into there
that should be more internal.
