Add locale.maxTaskPar, adjust dataParTasksPerLocale.

Add documentation for the locale.maxTaskPar method, and document that
the default value of dataParTasksPerLocale is the .maxTaskPar for the
top-level locale.
There's one just like this (in a parenthetical) in the very last line of Data_Parallelism.tex. That was where I got the idea. But maybe that one is wrong too and I need to update both? (easily done)