*: hide the config global-kill and session var tidb_enable_index_merge_join (#23395)

[ti-srebot]

cherry-pick #23395 to release-5.0
You can switch your code base to this Pull Request by using git-extras:

# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/23407

After apply modifications, you can push your change to this PR via:

git push [email protected]:ti-srebot/tidb.git pr/23407:release-5.0-901891ae1f44

---

What problem does this PR solve?

Problem Summary:

What is changed and how it works?

What's Changed:

The features global-kill and index merge join are experimental features, thus we need to hide the config and session variables.

How it Works:

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
  hide the config global-kill docs-cn#5770
• Need to cherry-pick to the release branch

Check List

Tests

• Unit test

Side effects

N/A

Release note

• N/A

[ti-srebot]

/run-all-tests

[ti-srebot]

@XuHuaiyu you're already a collaborator in bot's repo.

[jackysp]

/lgtm

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• jackysp
• lzmhhh123

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[XuHuaiyu]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 57e7df8

[ti-chi-bot]

@ti-srebot: Your PR was out of date, I have automatically updated it for you.

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[XuHuaiyu]

The unit-test failed because of the following case, it has nothing to do with this commit

1-03-18T11:45:45.598Z] FAIL: simple_test.go:606: testSuite3.TestDropPartitionStats
[2021-03-18T11:45:45.598Z] 
[2021-03-18T11:45:45.598Z] simple_test.go:631:
[2021-03-18T11:45:45.598Z]     tk.MustExec("analyze table t")
[2021-03-18T11:45:45.598Z] /home/jenkins/agent/workspace/tidb_ghpr_unit_test/go/src/github.com/pingcap/tidb/util/testkit/testkit.go:217:
[2021-03-18T11:45:45.598Z]     tk.c.Assert(err, check.IsNil, check.Commentf("sql:%s, %v, error stack %v", sql, args, errors.ErrorStack(err)))
[2021-03-18T11:45:45.598Z] ... value *errors.withStack = [stats]: some partition level statistics are not in statistics version 2, please set tidb_analyze_version to 2 and analyze the this table ("[stats]: some partition level statistics are not in statistics version 2, please set tidb_analyze_version to 2 and analyze the this table")
[2021-03-18T11:45:45.598Z] ... sql:analyze table t, [], error stack [stats]: some partition level statistics are not in statistics version 2, please set tidb_analyze_version to 2 and analyze the this table