[docs] Improve cluster-level docs

docs/usermanual-clusters.xml

@@ -182,8 +182,7 @@
     </para>
     <itemizedlist>
       <listitem>
-	<para><emphasis>Level 0</emphasis> is the default and
-	reproduces the behavior of the old HarfBuzz library.
+	<para><emphasis>Level 0</emphasis> is the default.
 	</para>
 	<para>
 	  The distinguishing feature of level 0 behavior is that, at
@@ -206,6 +205,11 @@
 	  url="https://www.unicode.org/reports/tr29/#Regex_Definitions">Unicode
 	  Technical Report 29</ulink>.
 	</para>
+	<para>
+	  This cluster level is suitable for code that likes to use
+	  HarfBuzz cluster values as an approximation of the Unicode
+	  Grapheme Cluster Boundaries as well.
+	</para>
 	<para>
 	  Client programs can specify level 0 behavior for a buffer by
 	  setting its <literal>cluster_level</literal> to
@@ -220,13 +224,13 @@
 	  implement backward compatibility with the old HarfBuzz.
 	</para>
 	<para>
-	  Level 1 differs from level 0 by not merging the 
+	  <emphasis>Level 1</emphasis> differs from level 0 by not merging the
 	  clusters of marks and other modifier code points with the
 	  preceding "base" code point's cluster. By preserving the
 	  separate cluster values of these marks and modifier code
 	  points, script shapers can perform additional operations
-	  that might lead to improved results (for example, reordering
-	  a sequence of marks).
+	  that might lead to improved results (for example, coloring
+	  mark glyphs differently than their base).
 	</para>
 	<para>
 	  Client programs can specify level 1 behavior for a buffer by