[analyzer] Add CTU user docs

Reviewers: dkrupp, a_sidorin, Szelethus, NoQ

Subscribers: whisperity, xazax.hun, baloghadamsoftware, szepet, rnkovacs, a.sidorin, mikhail.ramalho, donat.nagy, gamesh411, Charusso, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64801

llvm-svn: 366439