Coloring and Theming - Add closure doc

[NotTheDr01ds]

From this Discord comment from @sholderbach:

his never made it into the proper documentation but if the condition is only dependent on the individual value you can use: https://www.nushell.sh/blog/2022-12-20-nushell-0.73.html#more-sophisticated-coloring-of-data-values-using-closures-webbedspace

Worth updating Coloring and Theming in Nu

[fdncred]

Good point. I agree. I'm also curious if closures work for every datatype in regards to theming.

[Kissaki]

@fdncred I assume you replied to my comment that I deleted; I noticed that the linked release note has a warning about limited usability/effectiveness - only in specific contexts. I deleted the comment then because I hadn't taken that into account / didn't fully understand it yet.

I created updated documentation in PR #1773 which includes that caveat and my confirmed findings when testing it out.