[Docs] Move the list of CFI schemes down to CFI doc, and update it.

Use proper headling levels in CFI doc. Before that, all sections
were considered a subsection of "Introduction".

Reviewers: pcc, kcc

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D15237

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@254771 91177308-0d34-0410-b5e6-96231b3b80d8