ProcessorCount: Update documentation

Changes:
- Module documentation is sorted into a function description and a note
  section is added where cmake_host_system_information() is mentioned as
  an alternative.
- Module usage example is moved to a titled section at the end for
  better readability.
- Instead of mentioning the physical processors count, the logical
  processors is used as this is what is most likely wanted in most cases
  and also all the used commands in this module return this number.
- Function's code style is synced a bit across the code base.