[analyzer] Update alpha and potential checker documentation, esp. alpha.valist

Summary:
Move alpha.valist from potential to alpha since it was implemented in D15227

Cleanup some HTML comments, add a missing link

Reviewers: jordan_rose, zaks.anna

Subscribers: cfe-commits, xazax.hun

Differential Revision: https://reviews.llvm.org/D25663

llvm-svn: 284445