Add lowercase requirement to $taxonomy parameter of register_taxonomy()

[crstauf]

Reported by @bengreeley on User Contributed Notes:

I have feedback for the description of the parameter for $taxonomy. Both register_taxonomy() and register_post_type() have a known bug where the taxonomy or post type name needs to be lowercase to work properly (#16600).

[The documentation (​https://developer.wordpress.org/reference/functions/register_taxonomy/)] mentions lowercase in ‘Additional Parameter Information’, which is buried a bit deeper on the page. I’d recommend to update this page to include language such as “may only contain lowercase alphanumeric characters” in the main parameter section for $taxonomy, similar to the way it’s added for register_post_type() at ​https://developer.wordpress.org/reference/functions/register_post_type/

Since using anything but uppercase can lead to serious issues that have yet to be resolved I think making this more obvious would be beneficial to the end user.