Update to reflect changes related to number of threads.

Document the new symbolic string values 'MAX_PHYSICAL' and 'MAX_LOGICAL'
for CHPL_RT_NUM_THREADS_PER_LOCALE.  Change terminology to talk about
physical CPUs and logical CPUs instead of cores and hyperthreads.
Merge pull request #488 from gbtitus/doc-symbolic-CHPL_RT_NUM_THREADS_PER_LOCALE

Update to reflect changes related to number of threads.

Document the new symbolic string values 'MAX_PHYSICAL' and 'MAX_LOGICAL'
for CHPL_RT_NUM_THREADS_PER_LOCALE. Change terminology to talk about
physical CPUs and logical CPUs instead of cores and hyperthreads.

Not reviewed.