Improve the jerry port api documents. (#4977)

Notable changes:
  - Remove the comments in port impl, that's easily getting to in-consistence
  - Sync the jerryscript-port.h and 05.PORT-API.md
  - Fixes the invalid comment in port codes

JerryScript-DCO-1.0-Signed-off-by: Yonggang Luo luoyonggang@gmail.com

targets/os/riot/source/jerry-port.c

@@ -18,11 +18,6 @@
 
 #include "jerryscript-port.h"
 
-/**
- * Dummy function to get the time zone adjustment.
- *
- * @return 0
- */
 int32_t
 jerry_port_local_tza (double unix_ms)
 {
@@ -32,11 +27,6 @@ jerry_port_local_tza (double unix_ms)
   return 0;
 } /* jerry_port_local_tza */
 
-/**
- * Dummy function to get the current time.
- *
- * @return 0
- */
 double
 jerry_port_current_time (void)
 {