Add and use new proj4.register function

src/ol/proj/index.jsdoc

@@ -21,15 +21,14 @@
  * for the required projections. These definitions can be obtained from
  * {@link https://epsg.io/}, and are a JS function, so can be loaded in a script
  * tag (as in the examples) or pasted into your application.
- * The first time there is a request for a projection, either with a
- * {@link ol.proj.projectionLike} or directly with {@link ol.proj.get}, the
- * code will check if the Proj4js library and the necessary definition are
- * loaded; if so, it will register the appropriate {@link ol.proj.Projection}
- * object and add transform functions between the new projection and all the
- * existing ones. See examples/wms-image-custom-proj for an example of this.
- * Because the check for presence of the Proj4js library and the definition only
- * takes place on the first request for them, this means they can be loaded
- * dynamically as needed; for example, with user-supplied data where you don't
+ *
+ * After all required projection definitions are added to proj4's registry (by
+ * using `proj4.defs()`), simply call `register(proj4)` from the `ol/proj/proj4`
+ * package. Existing transforms are not changed by this function. See
+ * examples/wms-image-custom-proj for an example of this.
+ *
+ * Additional projection definitions can be registered with `proj4.defs()` any
+ * time. Just make sure to call `register(proj4)` again; for example, with user-supplied data where you don't
  * know in advance what projections are needed, you can initially load minimal
  * support and then load whichever are requested.
  *