patch 8.2.2829: some comments are not correct or clear

Problem:    Some comments are not correct or clear.
Solution:   Adjust the comments.  Add test for cursor position.

src/regexp_bt.c

@@ -4764,7 +4764,7 @@ regtry(
 
 /*
  * Match a regexp against a string ("line" points to the string) or multiple
- * lines ("line" is NULL, use reg_getline()).
+ * lines (if "line" is NULL, use reg_getline()).
  * Returns 0 for failure, number of lines contained in the match otherwise.
  */
     static long