docs(formatter): improvements to currency, date, filter, and formatter library

Author: naomiblack

lib/core/annotation_src.dart

@@ -547,8 +547,11 @@ abstract class DetachAware {
 }
 
 /**
- * Use @[Formatter] annotation to register a new formatter. A formatter is a class
- * with a [call] method (a callable function).
+ * Use the @[Formatter] class annotation to register a new formatter.
+ *
+ * A formatter is a pure function that performs a transformation on input data from an expression.
+ * For more on formatters in Angular, see the documentation for the
+ * [angular:formatter](#angular-formatter) library.
  *
  * Usage:
  *

lib/formatter/currency.dart

@@ -1,7 +1,7 @@
 part of angular.formatter_internal;
 
 /**
- * Formats a number as a currency (for example $1,234.56).
+ * Formats a number as a currency (for example 1,234.56$).
  *
  * When no currency symbol is provided, '$' is used. For more on formatters,
  * see the [angular:formatter](#angular-formatter) library.

lib/formatter/date.dart

@@ -23,7 +23,7 @@ part of angular.formatter_internal;
  *
  *
  * For more on explicit formatting of dates and date syntax, see the documentation for the
- * [DartFormat class](http://api.dartlang.org/docs/releases/latest/intl/DateFormat.html).
+ * [DartFormat class](https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/intl/intl.DateFormat).
  *
  */
 @Formatter(name:'date')