Calling `register_rest_route()` too early in the loading process has the potential to cause some unintentional problems and pitfalls. Because `register_rest_route()` calls `rest_get_server()` (which creates the `WP_REST_Server` instance), calling the function directly and/or before `rest_api_init` should be discouraged.
For example, if `register_rest_route ()` is called on `init`, the REST API server instance is set up (and all functions added to `rest_api_init` and other related hooks are invoked), even though the current request may not be a REST request. Also, if `register_rest_route()` is called even earlier (say, in an `mu-plugin` file), required endpoints may be missing since normal plugins have not yet been loaded and have not had a chance to register their own action hooks.
This adds a `_doing_it_wrong()` notice the first time `register_rest_route()` is called before `rest_api_init` in a request to encourage best practices for registering REST API routes.
Props kraftbj, desrosj, timothyblynjacobs.
Fixes#45265.
git-svn-id: https://develop.svn.wordpress.org/trunk@44568 602fd350-edb4-49c9-b593-d223f7449a82
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