Add comment to compiltest fn that finds rustc root (rust-lang#771)

Author: celinval

Diff for: src/tools/compiletest/src/header.rs

@@ -176,6 +176,9 @@ impl Config {
         }
     }
 
+    /// This function finds the root source of the repository by starting at the source base for
+    /// compiletest. It will then visit its parent folder and check if we found the root by
+    /// checking if it can find the compiletest `Cargo.toml` file in the path relative to the root.
     pub fn find_rust_src_root(&self) -> Option<PathBuf> {
         let mut path = self.src_base.clone();
         let path_postfix = Path::new("src/tools/compiletest/Cargo.toml");