auto-increment: change "continuous" to monotonic (#3584)

[ti-srebot]

cherry-pick #3584 to release-4.0

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What is changed, added or deleted? (Required)

The AUTO_INCREMENT doc used the word "continuous" when it should be "monotonic". Continuous means "without any abrupt changes", i.e. at a constant rate (which could be up or down). Auto_increment is not really constant - since restarts could make values jump quite a bit. So could requesting a new batch of cached values.

Monotonic means "either never decreases or never increases". Which is the property it guarantees on a per-server basis.

The language will need version-specific changes because it says "30 thousand by default", but it needs to be 30 thousand explicitly for 2.1 which does not support AUTO_ID_CACHE.

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

• master (the latest development version)
• v4.0 (TiDB 4.0 versions)
• v3.1 (TiDB 3.1 versions): will update in another PR because the file does not exist in the earlier three versions. You can check it at the website.
• 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):

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Have version specific changes
• Might cause conflicts

[ti-srebot]

@nullnotnil,Thanks for your review. However, LGTM is restricted to Reviewers or higher roles.See the corresponding SIG page for more information. Related SIGs: docs(slack).