Add documentation for log_grid_bins.
Add documentation for log_grid_bins.

mit-crpg/openmc,samuelshaner/openmc,johnnyliu27/openmc,paulromano/openmc,shikhar413/openmc,kellyrowland/openmc,smharper/openmc,liangjg/openmc,lilulu/openmc,shikhar413/openmc,wbinventor/openmc,amandalund/openmc,walshjon/openmc,walshjon/openmc,samuelshaner/openmc,samuelshaner/openmc,samuelshaner/openmc,mit-crpg/openmc,mjlong/openmc,smharper/openmc,johnnyliu27/openmc,smharper/openmc,lilulu/openmc,mit-crpg/openmc,paulromano/openmc,walshjon/openmc,shikhar413/openmc,wbinventor/openmc,walshjon/openmc,liangjg/openmc,amandalund/openmc,sxds/opemmc,liangjg/openmc,smharper/openmc,wbinventor/openmc,johnnyliu27/openmc,bhermanmit/openmc,lilulu/openmc,amandalund/openmc,mjlong/openmc,amandalund/openmc,paulromano/openmc,paulromano/openmc,johnnyliu27/openmc,shikhar413/openmc,sxds/opemmc,mit-crpg/openmc,bhermanmit/openmc,liangjg/openmc,wbinventor/openmc,kellyrowland/openmc