esp8266/modules/ntptime.py: Add comment about configuring NTP host.

The ability to change the host is a frequently requested feature, so
explicitly document how it can be achieved using the existing code.

See issues #2121, #4385, #4622, #5122, #5536.

ports/esp8266/modules/ntptime.py

@@ -10,6 +10,7 @@ except:
 # (date(2000, 1, 1) - date(1900, 1, 1)).days * 24*60*60
 NTP_DELTA = 3155673600
 
+# The NTP host can be configured at runtime by doing: ntptime.host = 'myhost.org'
 host = "pool.ntp.org"
 
 def time():