Support: Update documentation for Program functions

The docstrings were describing an older interface that has been replaced with
functions.

Also describe the performance characteristics of FindProgramByName() and
ExecuteAndWait() explaining when it's best to avoid them.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@196932 91177308-0d34-0410-b5e6-96231b3b80d8