Add comments about resolveUrl idiosyncrasies.

lib/mixins/element-mixin.html

@@ -627,6 +627,10 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
        * this element. This method will return the same URL before and after
        * bundling.
        *
+       * Note that this function performs no resolution for URLs that start
+       * with `/` (absolute URLs) or `#` (hash identifiers).  For general purpose
+       * URL resolution, use `window.URL`.
+       *
        * @param {string} url URL to resolve.
        * @param {string=} base Optional base URL to resolve against, defaults
        * to the element's `importPath`

lib/utils/resolve-url.html

@@ -20,6 +20,10 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
     let resolveDoc;
     /**
      * Resolves the given URL against the provided `baseUri'.
+     * 
+     * Note that this function performs no resolution for URLs that start
+     * with `/` (absolute URLs) or `#` (hash identifiers).  For general purpose
+     * URL resolution, use `window.URL`.
      *
      * @memberof Polymer.ResolveUrl
      * @param {string} url Input URL to resolve