Improve the documentation of the wp_insert_post tax_input parameter

[dingo_d]

In the current docblock (which is pulled into the official docs) it's not clear what the structure of the tax_input parameter should be.

I'd like to add a PR with an extra explanation.

The following PR expands on the tax_input parameter docblock.

From the short description, you may miss the subtle things while using this parameter unless you dive into the code.

Trac ticket: https://core.trac.wordpress.org/ticket/54264

[SergeyBiryukov]

In 51913:

Docs: Improve documentation for the tax_input parameter of wp_insert_post().

Follow-up to [10222], [13217], [33922].

Props dingo_d.
Fixes #54264.