improve documentation

[pkra]

from dpvc:

There are a couple of caveats:

    In subnumcases (and numcases), if there is a \label in a case, it must be the last thing in the text of the case (the second column) and must be outside of math mode.
    The material for the left= and right= in empheq should have less height and depth than the enclosed environment, otherwise the stretchy delimiters they contain will stretch to the size of the left- or right-hand material instead of the content, and the placement will probably be wrong.  You can always use \smash to accomplish this, but then the final result will have the height and depth of the enclosed environment, and you may need to add more spacing above and both the display to compensate.
    The cases package requires the empheq package, so both need to be loaded for cases to work.