[clangd] NFC: Document URIDistance

`URIDistance` constructor should mention that `Sources` must contain
*absolute paths*, not URIs. This is not very clear when looking at the
interface, especially given that `distance(...)` accepts `URI`, not an
absolute path which can give the wrong impression.

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D51691

llvm-svn: 341639