improve on rds resource and update docs

[ShiChangkuo]

• add HasChange volume.0.size check
• rename some functions
• ignore the flavor check
• change flavor in acc test
• update docs

What this PR does / why we need it:

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:

PR Checklist

• Tests added/passed.
• Documentation updated.
• Schema updated.

Acceptance Steps Performed

$ make testacc TEST='./huaweicloud' TESTARGS='-run TestAccRdsInstanceV3'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud -v -run TestAccRdsInstanceV3 -timeout 360m -parallel 4
=== RUN   TestAccRdsInstanceV3_basic
=== PAUSE TestAccRdsInstanceV3_basic
=== RUN   TestAccRdsInstanceV3_ha
=== PAUSE TestAccRdsInstanceV3_ha
=== CONT  TestAccRdsInstanceV3_basic
=== CONT  TestAccRdsInstanceV3_ha
--- PASS: TestAccRdsInstanceV3_ha (598.25s)
--- PASS: TestAccRdsInstanceV3_basic (1023.25s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud       1023.302s

$ make testacc TEST='./huaweicloud' TESTARGS='-run TestAccRdsReadReplicaInstance_basic'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud -v -run TestAccRdsReadReplicaInstance_basic -timeout 360m -parallel 4
=== RUN   TestAccRdsReadReplicaInstance_basic
=== PAUSE TestAccRdsReadReplicaInstance_basic
=== CONT  TestAccRdsReadReplicaInstance_basic

--- PASS: TestAccRdsReadReplicaInstance_basic (1400.39s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud       1400.435s