Document wt action

[nguyen-dows]

As per #300. This PR also removes an old notice from Windows Terminal 1.4 (it's been many years and many releases since then!)

[learn-build-service-prod[bot]]

Learn Build status updates of commit edb1f93:

:warning: Validation status: warnings

File	Status	Preview URL	Details
TerminalDocs/customize-settings/actions.md	:warning:Warning	View	Details

TerminalDocs/customize-settings/actions.md

• Line 4, Column 9: [Warning: author-not-found - See documentation] Invalid value for author: 'Christopher Nguyen' is not a valid GitHub ID.

For more details, please refer to the build report.

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[learn-build-service-prod[bot]]

Learn Build status updates of commit 2551714:

:white_check_mark: Validation status: passed

File	Status	Preview URL	Details
TerminalDocs/customize-settings/actions.md	:white_check_mark:Succeeded	View

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel