Card devexp_395 Added documentation for STI.

I added the cli docs and I did small flag cleaning:
* unified certain flags (namely `callbackUrl` -> `callbackURL` and `savetempdir` -> `saveTempDir`)
* added `forcePull` and `saveTempDir` to `usage` command, as these make sense there as well

@bparees @csrwng waiting for your comments.
Is this trying to say "whether a previous image exists" or is there some other basis for determining this?
Either way, I think we should explicitly list the fact that incremental requires a previous image (with the same name as the output name for this build).