Clusters are reversed based on the direction, not script

Fixes https://github.com/harfbuzz/harfbuzz/issues/1818

docs/usermanual-clusters.xml

@@ -156,14 +156,16 @@
       order.
     </para>
     <para>
-      For left-to-right scripts (LTR) and top-to-bottom scripts (TTB),
+      For buffers in the left-to-right (LTR)
+      or top-to-bottom (TTB) text flow direction,
       HarfBuzz will preserve the monotonic property: client programs
       are guaranteed that monotonically increasing initial cluster
       values will be returned as monotonically increasing final
       cluster values.
     </para>
     <para>
-      For right-to-left scripts (RTL) and bottom-to-top scripts (BTT),
+      For buffers in the right-to-left (RTL)
+      or bottom-to-top (BTT) text flow direction,
       the directionality of the buffer itself is reversed for final
       output as a matter of design. Therefore, HarfBuzz inverts the
       monotonic property: client programs are guaranteed that