Fix docs and parameter name on `wp_attachment_is()`

[flixos90]

The docs for the parameter $post_id of wp_attachment_is() is incorrect. While it has correct type definitions, the description is not accurate. Also I would think the parameter should rather be called $post and have null as default, similar to usage in other functions.

[SergeyBiryukov]

In 38068:

Docs: Correct $post parameter name and description for wp_attachment_is() and wp_attachment_is_image().

Props flixos90.
Fixes #37377.