Docs describing limitations of the cpp11-migrate Loop Convert Transform

Sam Panzer, author of loop convert, provided a list of limitations of the tool
to be documented. (Thanks Sam!)

The transform's limitations are now documented in the existing user doc.
Included are examples of the cases where the tool may change semantics.

Author: Jack Yang <jack.yang@intel.com>


git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@178034 91177308-0d34-0410-b5e6-96231b3b80d8