Add a bit of explanation of SingleBranchScheduler and AnyBranchScheduler.

Refs #1821.
Clarify custom renderable docs.
Merge pull request #624 from tomprince/bug1821

Add a bit of explanation of SingleBranchScheduler and AnyBranchScheduler.
Don't set build wait timer if build_wait_timeout>0
Merge branch 'patch-1' of git://github.com/Jc2k/buildbot