Simplify documentation of split-screen mode, avoiding abuse of []

Simplify documentation of split-screen mode, avoiding abuse of []
to sometimes mean "character set", which conflicts with the normal
meaning of "optional element" in manual pages.  While here, add a
few related clarifications and tweak a few details.
Triggered by a minor bug report from <trondd at kagu-tsuchi dot com>,
and by bentley@ subsequently pointing out the abuse of [].
Patch using input from jmc@, who also agreed with some previous versions.