Clarify the discrepancy between the default $scheme values in rest_url() and get_rest_url()

[DrewAPicture]

As pointed out by a user on the code reference, rest_url() and get_rest_url() both define different defaults for the $scheme parameters. rest_url() defaults to 'json', and get_rest_url() to 'rest', respectively.

It would be good to provide notices in both DocBlocks explaining the discrepancy.

[kadamwhite]

Update on out-of-band discussion: @rmccue has advocated we standardize on json as rest_url is called in more places. This would make this a functional change in addition to a docs change; while this has a small chance of introducing breaking behavior if somebody is hooking into filters and expecting the other value, our existing inconsistency makes it unlikely this is being used many places. I'll attempt to validate that to make sure we're not ignoring a real potential concern.

[kadamwhite]

Update on the previous update: after further investigation it turns out this occurrence of json was missed in #34300, and rest is the "correct" term. Some digging through the plugin directory did not immediately highlight anywhere this would introduce backwards compatibility issues, so I'm uploading a patch which changes the 'json' string to 'rest'.

[kadamwhite]

In 46233:

REST API: Consistently default URL scheme to "rest", not "json".

The "json" scheme is a holdover from the pre-merge plugin version of the REST API, and was corrected elsewhere in core in [34300]. Only "rest" is officially supported in set_url_scheme().

Props DrewAPicture, rmccue.
Fixes #45169.