[doc] Expand a little bit on Uniscribe compatibility

https://github.com/harfbuzz/harfbuzz/issues/3556#issuecomment-1130247124

docs/usermanual-opentype-features.xml

@@ -68,7 +68,7 @@
       used for shaping can be quite involved; HarfBuzz tries
       to be compatible with the OpenType Layout specification
       and, wherever there is any ambiguity, HarfBuzz attempts to replicate the
-      output of Microsoft's Uniscribe engine. See the <ulink
+      output of Microsoft's Uniscribe engine, to the extent that is feasible and desirable. See the <ulink
       url="https://docs.microsoft.com/en-us/typography/script-development/standard">Microsoft
       Typography pages</ulink> for more detail.
     </para>