docs(showcase): add note to textarea

[tigercosmos]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/NG-ZORRO/ng-zorro-antd/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[X] Documentation content changes
[ ] Application (the showcase website) / infrastructure changes
[ ] Other... Please describe:

Add note on textarea showcase in nzInput. Let the description be more complete.

r? @vthinkxie

[coveralls]

Coverage remained the same at 41.278% when pulling 526e2a8 on tigercosmos:input into 03b87c1 on NG-ZORRO:master.

[wilsoncook]

@tigercosmos More better to add the usage of nzRows parameter under the API section, could you?

[tigercosmos]

I think we can add at both "showcase" and "API ". We usually read showcase first, and go to read API if something are not clear.
How do you think?

[wilsoncook]

Yeah, provide more info. of usage about nzRows below in API is better

[tigercosmos]

sure

[coveralls]

Coverage remained the same at 41.268% when pulling 6bde480 on tigercosmos:input into a49a382 on NG-ZORRO:master.

[coveralls]

Coverage remained the same at 41.268% when pulling 6bde480 on tigercosmos:input into a49a382 on NG-ZORRO:master.

[tigercosmos]

thanks