fix typos and troubleshooting in areaDetector viewers doc

Fix a few typos in the areaDetector viewers documentation.

Remove from the areaDetector viewers documentation troubleshooting
section the statement that the EPICS_CA_MAX_ARRAY_BYTES environment
variable is not used by the ImageJ viewer.  This used to be true, but
it's no longer true as of areaDetector R1-9.