Add docs on internals of defaultUrlTransform

Related-to GH-904.

Author: wooorm

lib/index.js

@@ -412,6 +412,10 @@ function post(tree, options) {
 /**
  * Make a URL safe.
  *
+ * This follows how GitHub works.
+ * It allows the protocols `http`, `https`, `irc`, `ircs`, `mailto`, and `xmpp`,
+ * and URLs relative to the current protocol (such as `/something`).
+ *
  * @satisfies {UrlTransform}
  * @param {string} value
  *   URL.

readme.md

@@ -242,6 +242,10 @@ React node (`ReactNode`).
 
 Make a URL safe.
 
+This follows how GitHub works.
+It allows the protocols `http`, `https`, `irc`, `ircs`, `mailto`, and `xmpp`,
+and URLs relative to the current protocol (such as `/something`).
+
 ###### Parameters
 
 * `url` (`string`)