improve comment

runtime/Java/src/org/antlr/v4/runtime/atn/ATN.java

@@ -141,11 +141,20 @@ public class ATN {
 	 * <p>If {@code context} is {@code null}, it is treated as {@link ParserRuleContext#EMPTY}.</p>
 	 *
 	 * Note that this does NOT give you the set of all tokens that could
-	 * appear at a given token position.  This function always computes
-	 * what token(s) can come next specific to a single context (call stack),
-	 * which is what you want for error reporting and recovery.  Getting
-	 * all *possible* tokens given a partial input stream is a separate
-	 * computation. See https://github.com/antlr/antlr4/issues/1428
+	 * appear at a given token position in the input phrase.  In other words,
+	 * it does not answer:
+	 *
+	 *   "Given a specific partial input phrase, return the set of all tokens
+	 *    that can follow the last token in the input phrase."
+	 *
+	 * The big difference is that with just the input, the parser could
+	 * land right in the middle of a lookahead decision. Getting
+     * all *possible* tokens given a partial input stream is a separate
+     * computation. See https://github.com/antlr/antlr4/issues/1428
+	 *
+	 * For this function, we are specifying an ATN state and call stack to compute
+	 * what token(s) can come next and specifically: outside of a lookahead decision.
+	 * That is what you want for error reporting and recovery upon parse error.
 	 *
 	 * @param stateNumber the ATN state number
 	 * @param context the full parse context