docs(fgs): add note for 'app_agency' parameter of the function resource

[wuzhuanhong]

What this PR does / why we need it:
Add note for app_agency parameter of the huaweicloud_fgs_function resource.

Which issue this PR fixes:
(optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged)
fixes #xxx

Special notes for your reviewer:

Release note:

add note for app_agency parameter of the huaweicloud_fgs_function resource.

PR Checklist

• Tests added/passed.

make testacc TEST='./huaweicloud' TESTARGS='-run=TestAccSomethingV0_basic'
...
=== RUN   TestAccSomethingV0_basic
--- PASS: TestAccSomethingV0_basic (70.75s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud       70.796s

• Documentation updated.

• Schema updated.

• CheckDeleted.

  • a. During query operation (Read Context)
    aa. Resource not found
    >>>>>> Paste the screenshot here <<<<<<

  • b. During delete/disassociate/unbind operation (Delete Context)
    ba. Resource not found
    >>>>>> Paste the screenshot here <<<<<<