[lldb][doc] Document how our LLDB table gen initialized options

Summary: This patch adds documentation that should make it easier to migrate from using the old initializers to the table gen format.

Reviewers: jingham

Reviewed By: jingham

Subscribers: abidh, lldb-commits, JDevlieghere

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D64670

llvm-svn: 366083