Add a doc for missing key function and an error message referencing the doc.

Add a doc for missing key function and an error message referencing the doc.

Summary:
This is a common error, and because many people don't know what the key
function is, it is sometimes very confusing.

The doc was originally written by Brooks Moses and slightly edited by me.

Reviewers: MaskRay, espindola

Subscribers: emaste, llvm-commits, arichardson

Differential Revision: https://reviews.llvm.org/D55968

llvm-svn: 349941