doc: cex documentation

* NEWS, doc/bison.texi: Add documentation for conflict counterexample
generation.

NEWS

@@ -4,6 +4,30 @@ GNU Bison NEWS
 
 ** New features
 
+*** Counterexample Generation
+
+  When given `--report=counterexamples` or `-Wcounterexamples`, bison will
+  now output counterexamples for conflicts in the grammar.  These are
+  strings in the grammar which can be parsed in two ways due to the
+  conflict.  For example:
+
+    Example              exp '+' exp • '/' exp
+    First derivation     exp ::=[ exp ::=[ exp '+' exp • ] '/' exp ]
+    Second derivation    exp ::=[ exp '+' exp ::=[ exp • '/' exp ] ]
+
+  This is a shift/reduce conflict caused by none of the operators having
+  precedence, so the example can be parsed in the two ways shown.   When
+  bison cannot find an example that can be derived in two ways, it instead
+  generates two examples that are the same up until the dot:
+
+    First example        expr • ID $end
+    First derivation     $accept ::=[ s ::=[ a ::=[ expr • ] ID ] $end ]
+    Second example       expr • ID ',' ID $end
+    Second derivation    $accept ::=[ s ::=[ a ::=[ expr ::=[ expr • ID ',' ] ] ID ] $end ]
+
+  In these cases, the parser usually doesn't have enough lookahead to
+  differentiate the two given examples.
+
 *** File prefix mapping
 
   Bison learned a new argument, `--file-prefix-map OLD=NEW`.  Any file path