Specify inputTypes handled by EditContext (#101)

Define the set of `inputType`s that will be "handled" by [Handle Input For EditContext](https://w3c.github.io/edit-context/#handle-input-for-editcontext).

See also w3c/input-events#154

For normative changes, the following tasks have been completed:

 * [X] Editing WG resolution on the proposed changes, with at least two implementers participating and not objecting: #94 (comment)
   * [X] WebKit
   * [X] Chromium
   * [X] Gecko

 * [X] For browsers that are shipping the feature, implementation bugs are filed for the proposed changes (link to bug, or write "Implementing"/"Not Implementing"):
   * [X] WebKit: Not Implementing (https://bugs.webkit.org/show_bug.cgi?id=269922)
   * [X] Chromium (https://bugs.chromium.org/p/chromium/issues/detail?id=348590031)
   * [X] Gecko: Not Implementing (https://bugzilla.mozilla.org/show_bug.cgi?id=1904161)

Closes #94.

index.html

@@ -733,6 +733,30 @@ <h4><dfn class="export">Handle input for EditContext</dfn></h4>
  </ol>
  </div><!-- algorithm -->
 
+ <h4><dfn>EditContext-handled inputType</dfn></h4>
+ <div class="algorithm">
+ An <a href="https://www.w3.org/TR/uievents/#dom-inputevent-inputtype">inputType</a> is an
+ [=EditContext-handled inputType=] if it is one of the following:
+ <ul>
+ <li><code>insertText</code></li>
+ <li><code>insertTranspose</code></li>
+ <li><code>deleteWordBackward</code></li>
+ <li><code>deleteWordForward</code></li>
+ <li><code>deleteContent</code></li>
+ <li><code>deleteContentBackward</code></li>
+ <li><code>deleteContentForward</code></li>
+ </ul>
+ <div class="note">
+ The <a href="https://www.w3.org/TR/uievents/#dom-inputevent-inputtype">inputType</a>s handled by {{EditContext}}
+ are those which operate only on raw text. Other <a href="https://www.w3.org/TR/uievents/#dom-inputevent-inputtype">inputType</a>s
+ that depend on formats, clipboard/dragdrop, undo, or browser UI like spellcheck cannot be handled by
+ {{EditContext}} since {{EditContext}}'s state does not include these concepts. If an author wants their
+ application to handle those <a href="https://www.w3.org/TR/uievents/#dom-inputevent-inputtype">inputType</a>s,
+ they need to process them manually in a <a href="https://www.w3.org/TR/uievents/#event-type-beforeinput">beforeinput</a>
+ event handler.
+ </div>
+ </div>
+
  <h4><dfn>Update the EditContext</dfn></h4>
  <div class="algorithm">
  <dl>