sql-statements: improve alter index docs

[TomShawn]

What is changed, added or deleted? (Required)

This only applies to master, which features invisible indexes.

It improves the consistency of the docs for ALTER INDEX to follow the common format (including ## MySQL compatibility). There is some duplication in the page where it has examples, and then a section for invisible indexes with examples. Since the main use case for this statement is for invisible indexes, I merged the two together. The ## syntax section is also somewhat redundant with both the synposis and examples sections. So I've removed it.

The CREATE INDEX docs are also updated to remove a broken link, pointing to ALTER INDEX for invisible index docs. Really invisible indexes should have its own documentation page, with these pages just being for statement reference - but that's for a different task. The text also said that it was a "new feature introduced in MySQL 8.0" -- I think this text can be confusing to a casual reader, because it could imply that TiDB is somehow code-related to MySQL 8.0, which it is not.

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)
• 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: sql-statements: improve alter index docs docs#3586
• Other reference link(s):

Do your changes match any of the following descriptions?

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

[ran-huang]

Rest LGTM

[ti-srebot]

@ran-huang, @bb7133, @AilinKid, PTAL.

[ti-srebot]

@ran-huang, @bb7133, @AilinKid, PTAL.

[bb7133]

LGTM

[ti-srebot]

@bb7133,Thanks for your review. The bot only counts LGTMs from Reviewers and higher roles, but you're still welcome to leave your comments.See the corresponding SIG page for more information. Related SIG: docs(slack).