Update some comments

drivers/serial/Kconfig

@@ -2041,7 +2041,7 @@ config PSEUDOTERM_SUSV1
 	---help---
 		PTYs as specified in the Single Unix Specification (SUSv1).
 
-		Master: /dev/ptmx
+		Master: /dev/ptmx (multiplexor)
 		Slave: /dev/pts/N
 
 		Where N is the minor number

drivers/serial/ptmx.c

@@ -291,7 +291,7 @@ static ssize_t ptmx_write(FAR struct file *filep, FAR const char *buffer, size_t
  *   None
  *
  * Description:
- *   Register the master pseudo-terminal device at /dev/ptmx
+ *   Register the master pseudo-terminal multiplexor device at /dev/ptmx
  *
  * Returned Value:
  *   Zero (OK) is returned on success; a negated errno value is returned on

drivers/serial/pty.c

@@ -729,7 +729,7 @@ int pty_register(int minor)
   /* Register the master device
    *
    * BSD style (deprecated):  /dev/ptyN
-   * SUSv1 style: Master: /dev/ptmx (see ptmx.c)
+   * SUSv1 style: Master: /dev/ptmx (multiplexor, see ptmx.c)
    *
    * Where N is the minor number
    */

include/nuttx/serial/pty.h

@@ -61,7 +61,7 @@ extern "C"
  *   None
  *
  * Description:
- *   Register the master pseudo-terminal device at /dev/ptmx
+ *   Register the master pseudo-terminal multiplexor device at /dev/ptmx
  *
  * Returned Value:
  *   Zero (OK) is returned on success; a negated errno value is returned on