Fix typos in parallel doc
Fix inconsistencies in parallel direct interface doc

Running through the entire page would produce errors, as different
variable names were used, or, in the case of the %px a=b example, not
throw an exception because a and b actually were defined in an earlier
example.

The following fixes allow one to follow through the entire page and
experience the expected behaviors
Merge pull request #4823 from adamatus/parallel-doc-fixes

Minor fixes for typos/inconsistencies in parallel docs