Add user documentation for cpp11-migrate

Adding an RST document for cpp11-migrate. This user documentation explains
command line options, transformations, risk level and how it applies to
loop convert, and code examples of transformations.

There is a TODO task under "Risk" for "Loop Convert" to find code examples
that produce incorrect transformations that change semantics. The definition
of risk in loop convert and instances where the confidence level is lowered
will need to be looked at more carefully.

Information for all new features (including verbose output, auto transform)
will be added in a later change.

Author: Jack Yang <jack.wang@intel.com>
Reviewer: grigozavr


git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@176046 91177308-0d34-0410-b5e6-96231b3b80d8