Docs: clarify what an empty parent array means during block registration - #81525

What?

Closes #15731

Documents what an empty parent array means during block registration, and corrects the property's documented type.

Why?

The issue asks for the behaviour to be spelled out, and notes that Type: Array isn't quite right and that the default is unclear. The assumption in the issue — that an empty array restricts a block to the root — is the opposite of what actually happens, which is a good reason to write it down.

How?

canInsertBlockType passes blockType.parent to checkAllowList, which for an array returns list.includes( item ). For [] that is always false, including at the root where the item is null. Insertion is then only permitted when a container explicitly allows the block through its own allowedBlocks (hasParentAllowedBlock === true). So:

• omitted → no parent restriction;
• [] → matches no parent, including the root, so the block cannot be inserted anywhere and does not show in the inserter, unless a container lists it in allowedBlocks.

Documented that in block-metadata.md and block-registration.md, changed the documented type from Array to string[], noted the default, and mirrored the clarification in the block.json schema description and the BlockType type's doc comment.

Documentation only — no behaviour change.

Testing Instructions

1. Read the changed sections and check they match the behaviour described above.
2. To confirm the behaviour itself: register a test block with parent: [], then search for it in the inserter at the root and inside a Group — it is offered in neither. Add allowedBlocks: [ 'your/block' ] to a container and it becomes insertable there.
3. npm run docs:build (or npm run docs:check-api-docs-unstaged) should report no changes to regenerate.

Testing Instructions for Keyboard

n/a — documentation only.

Use of AI Tools

AI tooling (Claude Code) was used while investigating the issue and drafting the wording. I traced the behaviour through canInsertBlockType/checkAllowList myself before documenting it, and take responsibility for the content of this PR.