[docs][llvm-dwarfdump] Make the --show-parents and --show-children help text...

[docs][llvm-dwarfdump] Make the --show-parents and --show-children help text and docs more consistent and correct

The docs and help text for --show-parents and --show-children were a bit
inconsistent. The help text claimed they had an effect when "=<offset>"
was used, whereas the doc said it had an effect when "--find" or
"--name" were used. This change changes the doc to mention "=<offset>"
and removes this reference from the help text, to avoid having a very
long description in the help text (it still says "when selectively
printing entries").

Reviewed by: JDevlieghere, aprantl

Differential Revision: https://reviews.llvm.org/D63275

llvm-svn: 363380