Document unroll and unroll_count directives.

Extend the documentation for "#pragma clang loop" hints to include the unroll
and unroll_count directives.

Patch by Mark Heffernan [http://reviews.llvm.org/D4198]



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@211286 91177308-0d34-0410-b5e6-96231b3b80d8