update descriptions of expressions supported to be pushed down

[en-jin19]

What is changed, added or deleted? (Required)

• update descriptions of expressions supported to be pushed down in "functions-and-operators/expressions-pushed-down.md".
• revise the order of tidb_sysdate_is_now in "system-variables.md".

Which TiDB version(s) do your changes apply to? (Required)

Tips for choosing the affected version(s):

By default, CHOOSE MASTER ONLY so your changes will be applied to the next TiDB major or minor releases. If your PR involves a product feature behavior change or a compatibility change, CHOOSE THE AFFECTED RELEASE BRANCH(ES) AND MASTER.

For details, see tips for choosing the affected versions (in Chinese).

• master (the latest development version)
• v6.0 (TiDB 6.0 versions)
• v5.4 (TiDB 5.4 versions)
• v5.3 (TiDB 5.3 versions)
• v5.2 (TiDB 5.2 versions)
• v5.1 (TiDB 5.1 versions)
• v5.0 (TiDB 5.0 versions)
• v4.0 (TiDB 4.0 versions)
• v3.1 (TiDB 3.1 versions)
• v3.0 (TiDB 3.0 versions)
• v2.1 (TiDB 2.1 versions)

What is the related PR or file link(s)?

• This PR is translated from:
• Other reference link(s): revise expressions pushed down #8684

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

[TomShawn]

@en-jin19 请@ tech review 并尽早推进合并

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• Yisaer

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[en-jin19]

/merge

[ti-chi-bot]

@en-jin19: /merge in this pull request requires 2 approval(s).

In response to this:

/merge

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[en-jin19]

/merge

[ti-chi-bot]

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

Commit hash: d3084e3