Add more indirector documentation.

This adds:
 * general text about the indirector
 * the indirected class for each indirection
 * indirection-specific docs for a few indirections that I could manage to puzzle out
clarify what POST means
restore P:I:Indirection.initialize to its original signature
fix spec failures
Merge pull request #1559 from djmitche/indirdocs

Add more indirector documentation.