[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

clang-tools-extra/clangd/FileDistance.h

@@ -89,6 +89,7 @@ private:
 // comparison on the bodies.
 class URIDistance {
 public:
+  // \p Sources must contain absolute paths, not URIs.
   URIDistance(llvm::StringMap<SourceParams> Sources,
               const FileDistanceOptions &Opts = {})
       : Sources(Sources), Opts(Opts) {}