C++: Add QLDoc.

cpp/ql/src/definitions.qll

@@ -215,5 +215,10 @@ Top definitionOf(Top e, string kind) {
   strictcount(result.getLocation()) < 10
 }
 
+/**
+ * Returns an appropriately encoded version of a filename `name`
+ * passed by the VS Code extension in order to coincide with the
+ * output of `.getFile()` on locatable entities.
+ */
 cached
 File getEncodedFile(string name) { result.getAbsolutePath().replaceAll(":", "_") = name }