Changed thread-local to partition-local

[rpetrusha]

Changed thread-local to partition-local

Fixes #5412

@stephentoub, I'm wondering if changing the description here from thread-local to partition-local is such a good idea, in part because it should really be done to the other Parallel.For and ForEach methods with local variables. What do you think?

[BillWagner]

From a content perspective, this LGTM.

I'd also like @stephentoub 's opinion on these changes.

[stephentoub]

partition sounds fine to me. Arguably neither thread nor partition is 100% accurate, but being 100% accurate would entail implementation details that are subject to change. (In the current implementation, it's really a task-local value, where any number of tasks may be spun up by the implementation and which may run on that or a smaller number of threads and which may pull from any number of partitions of the input data.)