Adjusting coverage statement (#4148)

Updating the statement in understanding docs above each technique. From
#3024 (comment)

Co-authored-by: Mike Gower <[email protected]>

Author: alastc

_includes/understanding/intro/techniques.html

@@ -1,7 +1,7 @@
 <p>
   Each numbered item in this section represents a technique or combination of techniques
   that the WCAG Working Group deems sufficient for meeting this Success Criterion.
-  However, it is not necessary to use these particular techniques.
+  A technique may go beyond the minimum requirement of the criterion. There may be other ways of meeting the criterion not covered by these techniques. 
   For information on using other techniques, see
   <a href="understanding-techniques">Understanding Techniques for WCAG Success Criteria</a>,
   particularly the "Other Techniques" section.