Add parameter description

I think this should use MRs instead to make it easier for maintainers to review and merge.

I struggled for awhile trying to figure out how to specify that a param for a custom REST resource should be in the query.

I was able to do so using the patch from #15. (#14 worked for my purposes too.)

After applying the patch, I modified my @RestResource annotation to include the info about the query param like this:

/**
 * Provides a REST resource.
 *
 * @RestResource(
 *   id = "my_rest_resource",
 *   label = @Translation("My REST Reource"),
 *   uri_paths = {
 *     "create" = "/api/v1/foo/{path_param_1}/bar/{path_param_2}"
 *   },
 *   route_parameters = {
 *     "POST" = {
 *       "my_query_param" = {
 *         "name" = "my_query_param",
 *         "in" = "query",
 *         "type" = "string",
 *         "description" = "My custom query param in addition to the path params",
 *         "required" = true,
 *       }
 *     }
 *   }
 * )
 */

Setting status to Needs work for somebody to take @anybody's advice in turning this into a MR.

Sorry @immaculatexavier!
I totally missed that you had already made this a MR. :-p

MR !8 is working as well for my purpose of defining a query-based endpoint param.