[css-logical-props] Explain logical box model properties in a little more detail, and give them a proper toplevel section heading.

This adds an explanation of how logical and physical properties cascade
together.

It also adds an additional open issue about whether getComputedStyle()
should accept logical properties.  (In Gecko's current implementation,
I believe it does not do so, since it seemed easier to add later if
we want to add it than to remove later if we don't.)

This also makes the logical properties have their own toplevel section
rather than being a subsection of the logical page classifications,
which seems like it was a mistake.

I'm just committing this per
https://log.csswg.org/irc.w3.org/css/2015-07-21/#e573158

Author: dbaron

css-logical-props/Overview.bs

@@ -131,6 +131,66 @@ Logical Page Classifications</h2>
   The logical page selectors have specificity equal to the ':left'
   and ':right' page selectors.
 
+<h2 id="logical-box-props">
+Logical Box Model Properties
+</h2>
+
+  This specification introduces new CSS properties
+  that are logical equivalents
+  of physical box model properties.
+
+  The specified values of these properties are separate from
+  the specified values of the parallel physical properties,
+  but the logical and physical properties share computed values.
+  Which pairs of properties share computed values
+  depends on the computed values of
+  'writing-mode', 'text-orientation', and 'direction'.
+  For a summary of these dependencies, see
+  <a href="https://drafts.csswg.org/css-writing-modes/#logical-to-physical">Abstract-to-Physical Mappings</a>
+  in [[!CSS3-WRITING-MODES]].
+
+  A computed value that has logical and physical properties
+  is determined by applying the CSS cascade to declarations of both.
+  Overriding is not determined by whether a declaration is logical or physical,
+  but only by the rules of the CSS cascade [[!CSS3-CASCADE]].
+
+  <p class="note">
+    Note that this requires implementations to maintain
+    relative order of declarations within a CSS declaration block,
+    which was not previously required for CSS cascading.
+  </p>
+
+  <div class="example">
+    For example, given the following rule:
+
+    <pre class="lang-css">
+p {
+  margin-inline-start: 1px;
+  margin-left: 2px;
+  margin-inline-end: 3px;
+}
+    </pre>
+
+    In a paragraph with computed 'writing-mode' being ''horizontal-tb''
+    and computed 'direction' being ''ltr'',
+    the computed value of 'margin-left' is ''2px'',
+    since for that 'writing-mode' and 'direction',
+    'margin-inline-start' and 'margin-left' share a computed value,
+    and the declaration of 'margin-left'
+    is after the declaration of 'margin-inline-start'.
+    However, if the computed 'direction' were instead ''rtl'',
+    the computed value of 'margin-left' is ''3px'',
+    since 'margin-inline-end' and 'margin-left' share a computed value,
+    and the declaration of 'margin-inline-end'
+    is after the declaration of 'margin-left'.
+  </div>
+
+  <p class="issue">
+    How do computed value APIs, e.g., getComputedStyle() work?
+    Can they be used with either logical or physical properties,
+    or only with the physical ones?
+  </p>
+
 <h3 id="logical-dimension-properties">
 Logical Height and Logical Width: the 'block-size' and 'inline-size' properties</h3>