Document valid `type` parameters in register_meta() arguments

[iandunn]

The valid types aren't documented, so I had to go digging through the source to find them.

The REST API defines them as 'string', 'boolean', 'integer', 'number' in
class-wp-rest-meta-fields.php

I haven't verified that there aren't other valid types used outside the API.

[SergeyBiryukov]

The same list appears to apply to register_setting(): https://core.trac.wordpress.org/browser/tags/4.7.3/src/wp-includes/rest-api/endpoints/class-wp-rest-settings-controller.php?marks=263#L259

[priyankabehera155]

Hi @SergeyBiryukov

Added the valid types for the register_setting() function.

Thank you

[rmccue]

type was only introduced with the REST API in 4.7, so shouldn't be any other valid types outside of this.

Patches look good to me.

[rmccue]

In 40603:

Options/Meta: Document valid types for registration.

In 4.7, the REST API introduced types for options and meta passed into the registration functions. This adds the documentation for those types.

Props iandunn, priyankabehera155.
Fixes #40318.