doc: Separate pages into sub directories.

docs/api/browser/ipc-browser.md

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+# ipc (browser)
+
+The `ipc` module allows developers to send asynchronous messages to renderers.
+To avoid possible dead-locks, it's not allowed to send synchronous messages in
+browser.
+
+## Event: 'message'
+
+* `processId` Integer
+* `routingId` Integer
+
+Emitted when renderer sent a message to the browser.
+
+## Event: 'sync-message'
+
+* `event` Object
+* `processId` Integer
+* `routingId` Integer
+
+Emitted when renderer sent a synchronous message to the browser. The receiver
+should store the result in `event.result`.
+
+**Note:** Due to the limitation of `EventEmitter`, returning value in the
+event handler has no effect, so we have to store the result by using the
+`event` parameter.
+
+## ipc.send(processId, routingId, [args...])
+
+* `processId` Integer
+* `routingId` Integer
+
+Send `args...` to the renderer specified by `processId` and `routingId` and
+return immediately, the renderer should handle the message by listening to the
+`message` event.
+
+## ipc.sendChannel(processId, routingId, channel, [args...])
+
+* `processId` Integer
+* `routingId` Integer
+* `channel` String
+
+This is the same with ipc.send, except that the renderer should listen to the
+`channel` event. The ipc.send(processId, routingId, args...) can be seen as
+ipc.sendChannel(processId, routingId, 'message', args...).
+
+**Note:** If the the first argument (e.g. `processId`) is a `BrowserWindow`,
+`ipc.sendChannel` would automatically get the `processId` and `routingId`
+from it, so you can send a message to window like this:
+
+```javascript
+ipc.sendChannel(browserWindow, 'message', ...);
+```