docs($filter): clarify what is a valid filter name

Also updates the CHANGELOG to add info about this breaking some
previous invalid uses of $filter

Closes #10054
Closes #10131

Author: pkozlowski-opensource

CHANGELOG.md

@@ -94,7 +94,12 @@
   ([8eabc546](https://github.com/angular/angular.js/commit/8eabc5463c795d87f37e5a9eacbbb14435024061),
    [#9942](https://github.com/angular/angular.js/issues/9942))
 
+## Breaking Changes
 
+- **$parse:** due to [fbad2805](https://github.com/angular/angular.js/commit/fbad2805703569058a4a860747b0e2d8aee36bdf),
+    you can't use characters that have special meaning in AngularJS expressions (ex.: `.` or `-`)
+    as part of filter's name. Before this commit custom filters could contain special characters
+    (like a dot) in their name but this wasn't intentional.
 
 <a name="1.3.2"></a>
 # 1.3.2 cardiovasculatory-magnification (2014-11-07)

docs/content/guide/filter.ngdoc

@@ -91,6 +91,11 @@ The filter function should be a [pure function](http://en.wikipedia.org/wiki/Pur
 means that it should be stateless and idempotent. Angular relies on these properties and executes
 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.
+</div>
+
 The following sample filter reverses a text string. In addition, it conditionally makes the
 text upper-case.