docs: clarify strict_optional behavior

[Whning0513]

What does this PR do?

Clarifies the strict_optional configuration documentation:

• states that the config option disables optional checking only when set to false;
• replaces the unhelpful warning wording with a direct link to the explanation of optional types.

This keeps the config-file and command-line documentation consistent.

Fixes #21658

[Copilot]

Pull request overview

Clarifies strict_optional behavior and aligns configuration-file and command-line documentation.

Changes:

• Documents that optional checking is disabled only when set to false.
• Replaces warning text with explanatory links.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
docs/source/config_file.rst	Clarifies strict_optional = false behavior.
docs/source/command_line.rst	Updates warning guidance and links to further explanation.

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