[dsymutil][doc] Improve wording in manpage and rename file.

 - Improve wording
 - Rename llvm-dsymutil to dsymutil
 - Name -arch=<arch> argument

Differential revision: https://reviews.llvm.org/D39561

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@317226 91177308-0d34-0410-b5e6-96231b3b80d8
7 files changed