message: docs: update date pipe api reference to indicate it is a pure pipe
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diff: @@ -17,6 +17,14 @@ import {invalidPipeArgumentError} from './invalid_pipe_argument_error';
  *
  * Formats a date value according to locale rules.
  *
  + * `DatePipe` is executed only when it detects a pure change to the input value.
  + * A pure change is either a change to a primitive input value
  + * (such as `String`, `Number`, `Boolean`, or `Symbol`),
  + * or a changed object reference (such as `Date`, `Array`, `Function`, or `Object`).
  + *
  + * Note that mutating a `Date` object does not cause the pipe to be rendered again.
  + * To ensure that the pipe is executed, you must create a new `Date` object.
  + *
  * Only the `en-US` locale data comes with Angular. To localize dates
  * in another language, you must import the corresponding locale data.
  * See the [I18n guide](guide/i18n#i18n-pipes) for more information.
  