editorial: Reorganize internal slots into different sections (#156)

In #152, it was reported that the `[[PostureOverride]]` slot was confusing,
and readers could end up thinking that its `null` value could be exposed to
script, which is not the case (this internal slot is only used internally in
calculations that result in a DevicePostureType).

To avoid confusion:
- Rename the "Internal slots" section to "Extensions to the Document
  interface", which is similar to what we already have for the Navigator
  interface.
- Move the `[[PostureOverride]]` slot's definition and the addition of
  internal slots to top-level traversables to the Automation section, and
  add a sentence explaining the values are not exposed to script directly.

While here, do some proof-reading in the "Extensions to the Document
interface" section.

Fixes #152

index.html

@@ -156,12 +156,10 @@ <h2>
  </section>
  <section>
  <h3>
- Internal slots
+ Extensions to the `Document` interface
  </h3>
  <p>
  The following internal slots are added to the {{Document}} interface.
- As it name implies, it will house the value of the current posture the
- device is
  </p>
  <table class="simple">
  <thead>
@@ -180,43 +178,7 @@ <h3>
  <dfn data-dfn-for="Document">[[\CurrentPosture]]</dfn>
  </td>
  <td>
- The <dfn>current posture</dfn>.
- </td>
- </tr>
- </tbody>
- </table>
- <p>
- [=Top-level traversables=] must have the following internal slots:
- </p>
- <table class="simple">
- <thead>
- <tr>
- <th>
- Internal slot
- </th>
- <th>
- Description
- </th>
- </tr>
- </thead>
- <tbody>
- <tr>
- <td>
- <dfn data-dfn-for=
- "top-level traversable">[[\PostureOverride]]</dfn>
- </td>
- <td>
- Posture to override posture defined by hardware. Used in
- automation. Possible values:
- <ul>
- <li>"{{DevicePostureType/continuous}}"
- </li>
- <li>"{{DevicePostureType/folded}}"
- </li>
- <li>
- <code>null</code>: Used when posture is defined by hardware.
- </li>
- </ul>
+ The device's <dfn>current posture</dfn>.
  </td>
  </tr>
  </tbody>
@@ -793,6 +755,46 @@ <h2>
  commands=] that allows users to control the reported device posture and
  simulate a real device.
  </p>
+ <p>
+ To support the [=extension commands=] below and their integration with
+ [[[#algorithms]]], [=top-level traversables=] must have the following
+ internal slots:
+ </p>
+ <table class="simple">
+ <thead>
+ <tr>
+ <th>
+ Internal slot
+ </th>
+ <th>
+ Description
+ </th>
+ </tr>
+ </thead>
+ <tbody>
+ <tr>
+ <td>
+ <dfn data-dfn-for=
+ "top-level traversable">[[\PostureOverride]]</dfn>
+ </td>
+ <td>
+ Overrides the [=current posture=] provided by the hardware.
+ Possible values:
+ <ul>
+ <li>"{{DevicePostureType/continuous}}"
+ </li>
+ <li>"{{DevicePostureType/folded}}"
+ </li>
+ <li>
+ <code>null</code>: Not overriding. The hardware-provided
+ value will be used instead.
+ </li>
+ </ul>
+ These values are not exposed directly to script.
+ </td>
+ </tr>
+ </tbody>
+ </table>
  <h3>
  Extension Commands
  </h3>