Be consistent with our STUB/STUB+ terminology throughout circmgr/guardmgr

The following discussion from !2046 (merged) should be addressed:

• @nickm started a discussion: (+3 comments)

  IMO we should do these things in the docs:

  • Explain what a stub circuit is
  • Explain when we would choose STUB and when we would choose STUB+
  • Get our terminology uniform on "STUB+" vs "Extended". (But let's not do a big rename until we've decided.)