docs(compile/notassign): description for compile/notassign error

Closes #3459

docs/content/error/compile/noass.ngdoc

@@ -1,4 +0,0 @@
-@ngdoc error
-@name $compile:noass
-@fullName Non-Assignable Expression
-@description

docs/content/error/compile/nonassign.ngdoc

@@ -0,0 +1,36 @@
+@ngdoc error
+@name $compile:nonassign
+@fullName Non-Assignable Expression
+@description
+
+This error occurs when a directive defines an isolate scope property that support two-way data-binding (using the `=` mode in the {@link guide/directive#directivedefinitionobject  directive definition}) but the directive is used with an expression that is not-assignable.
+
+In order for the two-way data-binding to work, it must be possible to write new values back into the path defined with the expression.
+
+For example, given a directive:
+
+```
+myModule.directive('myDirective', function factory() {
+  return {
+    ...
+    scope: {
+      'bind': '=localValue'
+    }
+    ...
+  }
+});
+```
+
+Following are invalid uses of this directive:
+```
+<my-directive bind="1+2">    <!-- ERROR because `1+2=localValue` is an invalid statement -->
+<my-directive bind="myFn()"> <!-- ERROR because `myFn()=localValue` is an invalid statement -->
+```
+
+
+To resolve this error, always use path expressions with scope properties that are two-way data-bound:
+```
+<my-directive bind="some.property">
+<my-directive bind="some[3]['property']">
+```
+