docs(filters): clarify filter name restrictions

See angular#10122

Author: petebacondarwin

docs/content/guide/filter.ngdoc

@@ -92,8 +92,10 @@ means that it should be stateless and idempotent. Angular relies on these proper
 the filter only when the inputs to the function change.
 
 <div class="alert alert-warning">
-**Note:** filter names must be valid angular expression identifiers, such as `uppercase` or `orderBy`.
-Names with special characters, such as hyphens and dots, are not allowed.
+**Note:** Filter names must be valid angular {@link expression} identifiers, such as `uppercase` or `orderBy`.
+Names with special characters, such as hyphens and dots, are not allowed.  If you wish to namespace
+your filters, then you can use capitalization (`myappSubsectionFilterx`) or underscores
+(`myapp_subsection_filterx`).
 </div>
 
 The following sample filter reverses a text string. In addition, it conditionally makes the

src/loader.js

@@ -243,10 +243,17 @@ function setupModuleLoader(window) {
            * @ngdoc method
            * @name angular.Module#filter
            * @module ng
-           * @param {string} name Filter name.
+           * @param {string} name Filter name - this must be a valid angular expression identifier
            * @param {Function} filterFactory Factory function for creating new instance of filter.
            * @description
            * See {@link ng.$filterProvider#register $filterProvider.register()}.
+           *
+           * <div class="alert alert-warning">
+           * **Note:** Filter names must be valid angular {@link expression} identifiers, such as `uppercase` or `orderBy`.
+           * Names with special characters, such as hyphens and dots, are not allowed. If you wish to namespace
+           * your filters, then you can use capitalization (`myappSubsectionFilterx`) or underscores
+           * (`myapp_subsection_filterx`).
+           * </div>
            */
           filter: invokeLater('$filterProvider', 'register'),
 

src/ng/filter.js

@@ -20,6 +20,13 @@
  * Dependency Injected. To achieve this a filter definition consists of a factory function which is
  * annotated with dependencies and is responsible for creating a filter function.
  *
+ * <div class="alert alert-warning">
+ * **Note:** Filter names must be valid angular {@link expression} identifiers, such as `uppercase` or `orderBy`.
+ * Names with special characters, such as hyphens and dots, are not allowed. If you wish to namespace
+ * your filters, then you can use capitalization (`myappSubsectionFilterx`) or underscores
+ * (`myapp_subsection_filterx`).
+ * </div>
+ *
  * ```js
  *   // Filter registration
  *   function MyModule($provide, $filterProvider) {
@@ -101,6 +108,13 @@ function $FilterProvider($provide) {
    * @name $filterProvider#register
    * @param {string|Object} name Name of the filter function, or an object map of filters where
    *    the keys are the filter names and the values are the filter factories.
+   *
+   *    <div class="alert alert-warning">
+   *    **Note:** Filter names must be valid angular {@link expression} identifiers, such as `uppercase` or `orderBy`.
+   *    Names with special characters, such as hyphens and dots, are not allowed. If you wish to namespace
+   *    your filters, then you can use capitalization (`myappSubsectionFilterx`) or underscores
+   *    (`myapp_subsection_filterx`).
+   *    </div>
    * @returns {Object} Registered filter instance, or if a map of filters was provided then a map
    *    of the registered filter instances.
    */