[css-text-3] Add appendix summarizing plaintext copy-paste rules wrt white-space and text-transform.

Author: fantasai

css-text-3/Overview.bs

@@ -2359,8 +2359,22 @@ Text Processing Order of Operations</h2>
     <li><a href="#text-align-property">text alignment</a>
   </ol>
 
+<h2 class="no-num" id="plaintext">
+Appendix B: Conversion to Plaintext</h2>
+
+  <p>This appendix is normative for the purpose of plaintext copy-paste operations.
+
+  <p>When a CSS-rendered document is converted to a plaintext format,
+  it is expected that:
+  <ul>
+    <li>The 'text-transform' property has no effect.
+    <li>[[#white-space-phase-1]] is applied and any sequence of
+        <a>collapsible</a> spaces at the beginning of a <a>block</a>
+        or immediately following a <a>forced line break</a> is removed.
+  </ul>
+
 <h2 class="no-num" id="default-stylesheet">
-Appendix B: Default UA Stylesheet</h2>
+Appendix C: Default UA Stylesheet</h2>
 
   <p>This appendix is informative,
   and is to help UA developers to implement a default stylesheet for HTML,
@@ -2376,7 +2390,7 @@ Appendix B: Default UA Stylesheet</h2>
     with <kbd>[css-text]</kbd> in the subject line.</p>
 
 <h2 class="no-num" id="script-groups">
-Appendix C: Scripts and Spacing</h2>
+Appendix D: Scripts and Spacing</h2>
 
   <p><em>This appendix is normative.</em></p>
 
@@ -2426,7 +2440,7 @@ Appendix C: Scripts and Spacing</h2>
   They have different tolerances for space-justification vs inter-character justification,
   but both admit both.
 
-<h2 id="character-properties" class="no-num">Appendix D.
+<h2 id="character-properties" class="no-num">Appendix E.
 Characters and Properties</h2>
 
   <p>Unicode defines four codepoint-level properties that are referenced
@@ -2462,7 +2476,7 @@ Characters and Properties</h2>
     but take their other properties from the first combining character in the sequence.
   </ul>
 
-<h2 id="script-tagging" class="no-num">Appendix E.
+<h2 id="script-tagging" class="no-num">Appendix F.
 Tagging Content by Writing System</h2>
 
 	While most languages have a preferred writing system,