Exception handling docs: Describe landingpad clauses' meanings in more detail

The original text is very terse, so I've expanded on it.

Specifically, in the original text:

 * "The selector value is a positive number if the exception matched a
   type info" -- It wasn't clear that this meant "if the exception
   matched a 'catch' clause".

 * "If nothing is matched, the behavior of the program is
   `undefined`_."  -- It's actually implementation-defined in C++
   rather than undefined, as the new text explains.

llvm-svn: 202209