docs(module:modal): add open & close & setConfirmLoading property

[giscafer]

relate pr #125

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?

• Documentation content changes

What is the current behavior?

Issue Number: N/A

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[ ] No

Other information

[coveralls]

Coverage remained the same at 40.369% when pulling dc0f078 on giscafer:master into 01012e4 on NG-ZORRO:master.