open.2: Enhance rationale discussion for openat() and friends

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/open.2

@@ -1314,11 +1314,24 @@ of the directory prefix given to
 .BR open ()
 could be changed in parallel with the call to
 .BR open ().
+Suppose, for example, that we wish to create the file
+.I path/to/xxx.dep
+if the file
+.I path/to/xxx
+exists.
+The problem is that between the existence check and the file creation step,
+.I path
+or
+.I to
+(which might be symbolic links)
+could be modified to point to a different location.
 Such races can be avoided by
 opening a file descriptor for the target directory,
 and then specifying that file descriptor as the
 .I dirfd
-argument of
+argument of (say)
+.BR fstatat (2)
+and
 .BR openat ().
 
 Second,