Massage misc/qtchooser

• expand documentation of the tool used to manage the links; while we have only Qt5 in the tree the whole thing is rather moot, it's good to tidy up before Qt6 shows up on the horizon.
• update the documentation in the Makefile to reflect the current state of Uses/qt.mk

I've bumped PORTREVISION because the package contents will change
with the updated script, but there's no functional change at all.

SVN hooks have forced me to remove svn:executable from the
script file, but this doesn't change the resulting permissions
on the installed executable and script.

PR: 242905