chore(VPCEP): endpoint resource support new fields and fix some error

[deer-hang]

What this PR does / why we need it:

• endpoint resource support new fields and fix some documentation error
  • support new fields policy_statement and policy_document.
  • Improve the document to be compatible with the new fields.

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:

• Field policy_document was not tested due to insufficient testing conditions.

Release note:

PR Checklist

• Tests added/passed.

make testacc TEST='./huaweicloud/services/acceptance/vpcep' TESTARGS='-run TestAccVPCEndpoint_Basic'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud/services/acceptance/vpcep -v -run TestAccVPCEndpoint_Basic -timeout 360m -parallel 4
=== RUN   TestAccVPCEndpoint_Basic
=== PAUSE TestAccVPCEndpoint_Basic
=== CONT  TestAccVPCEndpoint_Basic
--- PASS: TestAccVPCEndpoint_Basic (249.33s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud/services/acceptance/vpcep     249.373s

make testacc TEST='./huaweicloud/services/acceptance/vpcep' TESTARGS='-run TestAccVPCEndpoint_Public'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud/services/acceptance/vpcep -v -run TestAccVPCEndpoint_Public -timeout 360m -parallel 4
=== RUN   TestAccVPCEndpoint_Public
=== PAUSE TestAccVPCEndpoint_Public
=== CONT  TestAccVPCEndpoint_Public
--- PASS: TestAccVPCEndpoint_Public (43.64s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud/services/acceptance/vpcep     43.682s

make testacc TEST='./huaweicloud/services/acceptance/vpcep' TESTARGS='-run TestAccVPCEndpoint_gatewayEndpoint'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud/services/acceptance/vpcep -v -run TestAccVPCEndpoint_gatewayEndpoint -timeout 360m -parallel 4
=== RUN   TestAccVPCEndpoint_gatewayEndpoint
=== PAUSE TestAccVPCEndpoint_gatewayEndpoint
=== CONT  TestAccVPCEndpoint_gatewayEndpoint
--- PASS: TestAccVPCEndpoint_gatewayEndpoint (234.39s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud/services/acceptance/vpcep     234.434s

• 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 <<<<<<

[github-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This PR has been approved by: @deer-hang

[jinyangyang222]

/lgtm