docstrings (and more) update post-target-refactor

target docs were obsolete and/or just plain wrong:
  claimed sources= param was a list of strings
  claimed targets= param was a list of targets

target method Page.register_wiki
was tagged (with @manual.builddict) for inclusion in
the BUILD dictionary; this is now a troll, you can't call
target methods in BUILD files.

Testing Done:
./build-support/bin/ci.sh

./build-support/bin/publish_docs.sh -o
searched BUILD dictionary for old phrases "list of targets", "(list of files)", "base.Target"; confirmed they're gone

Bugs closed: 138, 140

Reviewed at https://rbcommons.com/s/twitter/r/391/