doc/comedilib.ent: Update 'linux_version' and 'comedi_date'
doc/comedilib.ent: Update 'linux_version' and 'comedi_date'

Bump the 'linux_version' entity from '5.7' to '5.8' and the
'comedi_date' entity from '2020-07-31' to '2020-08-11'.  These are used
are used as version information for 'doc/drivers-linux.txt' and
'doc/drivers.txt'.  Those remain up-to-date.  The bump in version is
only due to an impending release of comedilib 0.12.0.

Linux-Comedi/comedilib,Linux-Comedi/comedilib,Linux-Comedi/comedilib,Linux-Comedi/comedilib,Linux-Comedi/comedilib,Linux-Comedi/comedilib