Updates for parallel docs

I've made some updates to the parallel docs. (#3094)

- Clarify in the definition of ``pmap`` that one process is dedicated to assigning tasks. Previously it could be (incorrectly) assumed that there should be a speedup from 1 to 2 processes.
- Update references to deprecated functions.
- processors -> processes. I think this is clearer since we're talking about OS-level processes, not CPUs.
- Sync ``pmap`` with the version in the manual.
- Updates to the text describing ``pmap`` in the manual.