Doxygen improvements for sorc/grid_tools.fd/regional_esg_grid.fd/pmat…

….f90 (#382)

* Part of #366
Doxygen improvements for sorc/grid_tools.fd/regional_esg_grid.fd/pmat.f90

* Part of #366
Doxygen improvements for sorc/grid_tools.fd/regional_esg_grid.fd/pmat2.f90

* Doxygen improvements for pmat.f90

* Part of #366
Doxygen improvements for sorc/grid_tools.fd/regional_esg_grid.fd/pmat.f90

* fixed doxygen warnings

* punctuation

Co-authored-by: Edward Hartnett <Edward.Hartnett@noaa.gov>
Co-authored-by: Edward Hartnett <38856240+edwardhartnett@users.noreply.github.com>