Now that proto2 is defined, replaced XXX blocks with text about it.
Now that proto2 is defined, replaced XXX blocks with text about it.

Also moved the proto2 opcode descriptors into the sections they fit (like
TUPLE{1,2,3} in the group of tuple-building opcodes; etc).

sk-/python2.7-type-annotator,sk-/python2.7-type-annotator,sk-/python2.7-type-annotator