Minor documentation cleanup

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@185672 91177308-0d34-0410-b5e6-96231b3b80d8
Add some more documentation on how to navigate from a LambdaExpr::Capture
to the associated FieldDecl.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@185674 91177308-0d34-0410-b5e6-96231b3b80d8