chore: update docs and acceptance tests of ELB

[ShiChangkuo]

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="./flexibleengine/acceptance" TESTARGS="-run TestAccElbLogTank_basic"
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./flexibleengine/acceptance -v -run TestAccElbLogTank_basic -timeout 720m
=== RUN   TestAccElbLogTank_basic
=== PAUSE TestAccElbLogTank_basic
=== CONT  TestAccElbLogTank_basic
--- PASS: TestAccElbLogTank_basic (272.37s)
PASS
ok      github.com/FlexibleEngineCloud/terraform-provider-flexibleengine/flexibleengine/acceptance      272.411s

$ make testacc TEST="./flexibleengine/acceptance" TESTARGS="-run TestElbAccSecurityPolicie_basic"
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./flexibleengine/acceptance -v -run TestElbAccSecurityPolicie_basic -timeout 720m
=== RUN   TestElbAccSecurityPolicie_basic
=== PAUSE TestElbAccSecurityPolicie_basic
=== CONT  TestElbAccSecurityPolicie_basic
--- PASS: TestElbAccSecurityPolicie_basic (88.06s)
PASS
ok      github.com/FlexibleEngineCloud/terraform-provider-flexibleengine/flexibleengine/acceptance      88.108s