faq, sql-statements: update out of date info (#3418)

[ti-srebot]

cherry-pick #3418 to release-2.1

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What is changed, added or deleted? (Required)

This fixes some small errors in the FAQ and sql-statements guide:
-The FAQ implied optimistic locking was still the default, and said SELECT FOR UPDATE does not lock.

• The wording on sql-mode changing was a bit cryptic.
• The SET sql statement now points out differences from MySQL.

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)

This will require backporting, but the following version specific changes apply:

• default locking in 2.1 is optimistic, so the section SELECT FOR UPDATE will need rewording.
• SQL Plan Management is a 4.0+ feature
• coprocessor cache is a 4.0+ feature

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

[TomShawn]

@nullnotnil Please apply v2.1-specific changes in this PR and delete /faq/sql-faq.md.

[ghost]

@TomShawn Done

[TomShawn]

LGTM

[ti-srebot]

/run-all-tests