docs: correcting some aspects on Viewport resizing

[mfranzke]

Correct the documentation both on the config value and that it‘s not obsolete:

The second entry in ishViewportRange.l is the ishViewportMaximum, which is now also obsolete.

Actually the configuration name ishViewportMaximum needs to get corrected to ishMaximum

Originally posted by @mfranzke in #1145 (comment)