Add Calibre’s meta related details in EPUB compat doc

docs/CSS21-epub_compat.md

@@ -139,13 +139,15 @@ This item indicates the series the publication is part of.
 
 This item designates the position (index) of the publication in this series.
 
+It can be a floating point number with up to two digits of precision e.g. `1.01`, and zero and negative numbers are allowed.
+
 #### Rating
 
 ```
 <meta content="10.0" name="calibre:rating" />
 ```
 
-This item stores the rating the user has explicitely set for the publication. `2.0` is one star, `10.0` is five.
+This item stores the rating (integer) the user has explicitely set for the publication. In Calibre, `0.0` is unrated, `2.0` is one star, and `10.0` is five stars.
 
 ### iBooks
 

docs/ReadiumCSS_docs/OEBPS/Text/Section-021.xhtml

@@ -168,14 +168,16 @@
           <pre><code>&lt;meta content="1.0" name="calibre:series_index" /&gt;</code></pre>
 
           <p>This item designates the position (index) of the publication in this series.</p>
+
+          <p>It can be a floating point number with up to two digits of precision e.g. <code>1.01</code>, and zero and negative numbers are allowed.</p>
         </section>
 
         <section id="rating" class="level4">
           <h4 class="sigil_not_in_toc">Rating</h4>
 
           <pre><code>&lt;meta content="10.0" name="calibre:rating" /&gt;</code></pre>
 
-          <p>This item stores the rating the user has explicitely set for the publication. <code>2.0</code> is one star, <code>10.0</code> is five.</p>
+          <p>This item stores the rating (integer) the user has explicitely set for the publication. In Calibre, <code>0.0</code> is unrated, <code>2.0</code> is one star, and <code>10.0</code> is five stars.</p>
         </section>
       </section>