[asan_symbolize] Move argument parser epilog text into script doc comment.

Summary:
This will make it easier to expand on the documentation in the future
that avoids cluttering the code.

rdar://problem/49476995

Reviewers: kubamracek, yln, samsonov, dvyukov, vitalybuka

Subscribers: #sanitizers, llvm-commits

Tags: #llvm, #sanitizers

Differential Revision: https://reviews.llvm.org/D60398

llvm-svn: 357978