Bindings documentation does not mention limitation of protected post meta

[andreawetzel]

The block bindings documentation should note that post meta values that begin with an underscore cannot be used in the Block Bindings API. It also mention that the post meta must be available via the REST API.

PR where this security measure was added: #59326

Suggested edit

## Excluded Post Meta Values

Post meta with keys that begin with an underscore (such as `_example_key`) are considered protected and cannot be used to bind to a core block.

Also, post meta that is unavailable through the REST API cannot be used in the Block Bindings API.

Documentation URL: https://developer.wordpress.org/block-editor/reference-guides/block-api/block-bindings/
Github URL: https://github.com/WordPress/gutenberg/blob/trunk/docs/reference-guides/block-api/block-bindings.md