docs: Fixing links to moved Fragments guide, + redirect

The Fragments API guide was moved, but apparently a redirect was
never set up. Also, there are a few links to the old location in
the Javadocs.

Staged the revised Javadocs (see first comment for stage location).
Not going to stage the redirects file since it would trash another,
bigger CL that deals with the redirects file, but it's pretty
straightforward.

bug: 30559011
Change-Id: Ibd65f85c1ebb9789c1d40614fe11fe4ffda97e58