[docs][llvm-dwarfdump] Make some option descriptions clearer and more precise

Some of the wording in the doc (taken largely from the help text), was a
little imprecise in some cases, so this patch makes it a little more
precise.

Reviewed by: JDevlieghere, probinson

Differential Revision: https://reviews.llvm.org/D64332

llvm-svn: 365451