Description of proposed changes

See #5573 for background. This PR does a few things:

1. Adds +qquality as a modifier to psconvert -Tj so one can change the JPEG quality [Default remains at 90].
2. Documents that the default is 90.
3. Allows both the existing modifier +m as well as +q to be recognized when used in begin and figure (previously one could not pass +m in these modules).

All tests still pass, and this simple example shows both +m and +q being processed:

gmt begin map jpg+q95+m,pdf A,S
	gmt coast -RFR -JM6i -Gred -B
gmt end show