Rename and document some vaguely svcreq-related things

Rename and document some vaguely svcreq-related things

Summary: This diff contains no functional changes, just tidying up and
documentation in the wake of killing arch-dependent service requests.
Changes include:

- s/kJmpccLen/kJccLen/g

- Use `target' as the name of the to-be-translated SrcKey in all
  svcreq-related structures (namely, Vinstrs and ExtraDatas) instead of
  waffling between `target', `dest', and `sk'.
- Use a consistent order for service requests everywhere they're used
  (this will probably not last, but w/e).

- Organize and document the vasm intrinsics better.
- Kill vret (it is never used), then rename vretm to vret.
- Rename callstub to callarray; it sounds similar to callfaststub but
  actually isn't.

- Document method cache priming coherently in one location.

Reviewed By: @edwinsmith

Differential Revision: D2291378