docs(common): mark all common apis except for i18n as stable

modules/@angular/common/src/pipes/date_pipe.ts

@@ -80,7 +80,7 @@ var defaultLocale: string = 'en-US';
  *
  * {@example core/pipes/ts/date_pipe/date_pipe_example.ts region='DatePipe'}
  *
- * @experimental
+ * @stable
  */
 @Pipe({name: 'date', pure: true})
 export class DatePipe implements PipeTransform {

modules/@angular/common/src/pipes/lowercase_pipe.ts

@@ -18,7 +18,7 @@ import {InvalidPipeArgumentException} from './invalid_pipe_argument_exception';
  *
  * {@example core/pipes/ts/lowerupper_pipe/lowerupper_pipe_example.ts region='LowerUpperPipe'}
  *
- * @experimental
+ * @stable
  */
 @Pipe({name: 'lowercase'})
 export class LowerCasePipe implements PipeTransform {

modules/@angular/common/src/pipes/number_pipe.ts

@@ -86,7 +86,7 @@ function formatNumber(
  *
  * {@example core/pipes/ts/number_pipe/number_pipe_example.ts region='NumberPipe'}
  *
- * @experimental
+ * @stable
  */
 @Pipe({name: 'number'})
 export class DecimalPipe implements PipeTransform {
@@ -112,7 +112,7 @@ export class DecimalPipe implements PipeTransform {
  *
  * {@example core/pipes/ts/number_pipe/number_pipe_example.ts region='PercentPipe'}
  *
- * @experimental
+ * @stable
  */
 @Pipe({name: 'percent'})
 export class PercentPipe implements PipeTransform {
@@ -143,7 +143,7 @@ export class PercentPipe implements PipeTransform {
  *
  * {@example core/pipes/ts/number_pipe/number_pipe_example.ts region='CurrencyPipe'}
  *
- * @experimental
+ * @stable
  */
 @Pipe({name: 'currency'})
 export class CurrencyPipe implements PipeTransform {

modules/@angular/common/src/pipes/uppercase_pipe.ts

@@ -17,7 +17,7 @@ import {InvalidPipeArgumentException} from './invalid_pipe_argument_exception';
  *
  * {@example core/pipes/ts/lowerupper_pipe/lowerupper_pipe_example.ts region='LowerUpperPipe'}
  *
- * @experimental
+ * @stable
  */
 @Pipe({name: 'uppercase'})
 export class UpperCasePipe implements PipeTransform {