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.
diff --git a/src/regexp_nfa.c b/src/regexp_nfa.c
index 9edca81..a519b85 100644
--- a/src/regexp_nfa.c
+++ b/src/regexp_nfa.c
@@ -7287,7 +7287,7 @@
 
 /*
  * 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.
  */