Add some more SourcePath docs

src/libfetchers/input-accessor.hh

@@ -125,9 +125,17 @@ struct SourcePath
     std::string to_string() const
     { return path.abs(); }
 
+    /**
+     * Append a `CanonPath` to this path.
+     */
     SourcePath operator + (const CanonPath & x) const
     { return {path + x}; }
 
+    /**
+     * Append a single component `c` to this path. `c` must not
+     * contain a slash. A slash is implicitly added between this path
+     * and `c`.
+     */
     SourcePath operator + (std::string_view c) const
     {  return {path + c}; }