docs($interpolate): add known issue about end-symbol in expression

src/ng/interpolate.js

@@ -216,6 +216,11 @@ function $InterpolateProvider() {
      *  </file>
      * </example>
      *
+     * @knownIssue
+     * It is currently not possible for an interpolated expression to contain the interpolation end
+     * symbol. For example, `{{ '}}' }}` will be incorrectly interpreted as `{{ ' }}` + `' }}`, i.e.
+     * an interpolated expression consisting of a single-quote (`'`) and the `' }}` string.
+     *
      * @param {string} text The text with markup to interpolate.
      * @param {boolean=} mustHaveExpression if set to true then the interpolation string must have
      *    embedded expression in order to return an interpolation function. Strings with no