Add documentation for keyboard shortcut selecting text across multiple blocks

[bengreeley]

What?

Addresses #41168, implementing documentation for the keyboard shortcut for selecting text across multiple blocks (without selecting all of a blocks' text). Feature was originally implemented in #38892.

How?

Adds missing documentation on selecting text between blocks to editor 'Keyboard shortcuts' modal and the 'Are there keyboard shortcuts for Gutenberg' documentation.

Testing Instructions

1. Open Gutenberg editor
2. Press shift+alt+h to open Keyboard Shortcuts modal or open the modal by clicking the 3 vertical dots menu and selecting 'Keyboard shortcuts'
3. Verify the section about selecting text between multiple blocks is added.
4. Open the 'Are there keyboard shortcuts for Gutenberg' doc and verify new section about selecting text between multiple blocks has been added.

[github-actions]

👋 Thanks for your first Pull Request and for helping build the future of Gutenberg and WordPress, @bengreeley! In case you missed it, we'd love to have you join us in our Slack community, where we hold regularly weekly meetings open to anyone to coordinate with each other.

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[DaisyOlsen]

LGTM :-)

[aaronrobertshaw]

Thank you for updating these docs @bengreeley 👍

The new sections appear as advertised for me. I have a couple of small nits that I've left as inline comments. I believe with those tweaks the wording will be a touch more accurate and this will be ready to merge.

What do you think?

[aaronrobertshaw]

Apologies @bengreeley, this one slipped through the cracks of my notifications backlog 😅

I've retested this locally, after rebasing on trunk, and the documentation for selecting text across multiple blocks appears in both locations as advertised.

We might need to rebase this PR branch if re-running the failed tests doesn't get them passing. The failures appear to be unrelated to the changes in this PR.

[bengreeley]

Thanks for rebasing @aaronrobertshaw! It's been on my to-do list as well but haven't gotten around to it. Appreciate you taking a look!

[aaronrobertshaw]

I've rebased this PR and will merge once the tests pass

[github-actions]

Warning: Type of PR label error

To merge this PR, it requires exactly 1 label indicating the type of PR. Other labels are optional and not being checked here.

• Type-related labels to choose from: [Type] Accessibility (a11y), [Type] Automated Testing, [Type] Breaking Change, [Type] Bug, [Type] Build Tooling, [Type] Code Quality, [Type] Copy, [Type] Developer Documentation, [Type] Enhancement, [Type] Experimental, [Type] Feature, [Type] New API, [Type] Task, [Type] Performance, [Type] Project Management, [Type] Regression, [Type] Security, [Type] WP Core Ticket, Backport from WordPress Core.
• Labels found: [Type] Enhancement, [Type] Developer Documentation, First-time Contributor.

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