cmdutil: comprehensively document the interface of export
I want to make some improvements here, but in order to make future
patches easier to review I want to document the current state of the
world.
pager.pager=p0 # $TESTTMP/sysrc:4
ui.editor=e1 # $TESTTMP/userrc:2
pager.pager=p2 # $PAGER
ui.editor=e1 # $TESTTMP/userrc:2