linux ttyname: Update a reference to kernel docs for kernel 4.10

Linux 4.10 moved many of the documentation files around.

4.10 came out between the time the patch adding the comment (commit
15e9a4f378) was submitted and the time
it was applied (in February, January, and March 2017; respectively).

Reviewed-by: Christian Brauner <christian.brauner@ubuntu.com>

sysdeps/unix/sysv/linux/ttyname.h

@@ -21,7 +21,8 @@
 #include <sys/stat.h>
 
 /* Return true if this is a UNIX98 pty device, as defined in
-   linux/Documentation/devices.txt.  */
+   linux/Documentation/devices.txt (on linux < 4.10) or
+   linux/Documentation/admin-guide/devices.txt (on linux >= 4.10).  */
 static inline int
 is_pty (struct stat64 *sb)
 {