ui/input: expand package documentation

Signed-off-by: Elias Naur <mail@eliasnaur.com>

ui/input/input.go

@@ -1,7 +1,33 @@
 // SPDX-License-Identifier: Unlicense OR MIT
 
-// Package input exposes a unified interface to input sources. Subpackages
-// such as pointer and key provide the interfaces for specific input types.
+/*
+Package input exposes a unified interface for receiving input
+events.
+
+For example:
+
+	var queue input.Queue = ...
+
+	for e, ok := queue.Next(h); ok; e, ok = queue.Next(h) {
+		switch e.(type) {
+			...
+		}
+	}
+
+In general, handlers must be declared before events become
+available. Other packages such as pointer and key provide
+the means for declaring handlers for specific input types.
+
+The following example marks a handler ready for key input:
+
+	import gioui.org/ui/input
+	import gioui.org/ui/key
+
+	var ops ui.Ops
+	var h *Handler = ...
+	key.HandlerOp{Key: h}.Add(ops)
+
+*/
 package input
 
 // Queue maps an event handler key to the events