Update particle docs for expanded idcpu range

[atmyers]

I am making a series of PRs that update the particle documentation. This first one explains the current convention re: particle id and cpu numbers.

The proposed changes:

• fix a bug or incorrect behavior in AMReX
• add new capabilities to AMReX
• changes answers in the test suite to more than roundoff level
• are likely to significantly affect the results of downstream AMReX users
• include documentation in the code and/or rst files, if appropriate