[docs] #2174 docs and change log

docs/change-log.md

@@ -24,6 +24,8 @@
 - API: `m.mount()` will only render its own root when called, it will not trigger a `redraw()` ([#1592](https://github.com/MithrilJS/mithril.js/pull/1592))
 - API: Assigning to `vnode.state` (as in `vnode.state = ...`) is no longer supported. Instead, an error is thrown if `vnode.state` changes upon the invocation of a lifecycle hook.
 - API: `m.request` will no longer reject the Promise on server errors (eg. status >= 400) if the caller supplies an `extract` callback. This gives applications more control over handling server responses.
+- hyperscript: when attributes have a `null` or `undefined` value, they are treated as if they were absent. [#1773](https://github.com/MithrilJS/mithril.js/issues/1773) ([#2174](https://github.com/MithrilJS/mithril.js/pull/2174))
+- hyperscript: when an attribute is defined on both the first and second argument (as a CSS selector and an `attrs` field, respectively), the latter takes precedence, except for `class` attributes that are still added together. [#2172](https://github.com/MithrilJS/mithril.js/issues/2172) ([#2174](https://github.com/MithrilJS/mithril.js/pull/2174))
 
 #### News
 

docs/hyperscript.md

@@ -5,6 +5,7 @@
 - [How it works](#how-it-works)
 - [Flexibility](#flexibility)
 - [CSS selectors](#css-selectors)
+- [Attributes passed as the second argument](attributes-passed-as-the-second-argument)
 - [DOM attributes](#dom-attributes)
 - [Style attribute](#style-attribute)
 - [Events](#events)
@@ -144,7 +145,23 @@ m("a.link[href=/]", {
 // <a href="/" class="link selected">Home</a>
 ```
 
-If there are class names in both first and second arguments of `m()`, they are merged together as you would expect.
+### Attributes passed as the second argument
+
+You can pass attributes, properties, events and lifecycle hooks in the second, optional argument (see the next sections for details).
+
+```JS
+m("button", {
+	class: "my-button",
+	onclick: function() {/* ... */},
+	oncreate: function() {/* ... */}
+})
+```
+
+If the value of such an attribute is `null` or `undefined`, it is treated as if the attribute was absent.
+
+If there are class names in both first and second arguments of `m()`, they are merged together as you would expect. If the value of the class in the second argument is `null`or `undefined`, it is ignored.
+
+If another attribute is present in both the first and the second argument, the second one takes precedence even if it is  is `null` or `undefined`.
 
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