Add description of KEYD_DEBUG

docs/keyd-application-mapper.scdoc

@@ -75,6 +75,11 @@ which manages the script lifecycle.
 In order for this to work, keyd must be running and the user must have access
 to */var/run/keyd.socket* (i.e be a member of the *keyd* group). 
 
+# ENVIRONMENT VARIABLES
+
+*KEYD_DEBUG*
+	Debug log level. _0_,_1_ can be specified (default: 0).
+
 # A note on security
 
 *Any user which can interact with a program that directly controls input devices

docs/keyd.scdoc

@@ -959,6 +959,11 @@ Disables the esc and end keys.
 	end = noop
 ```
 
+# ENVIRONMENT VARIABLES
+
+*KEYD_DEBUG*
+	Debug log level. _0_,_1_,_2_ can be specified (default: 0).
+
 # AUTHOR
 
 Written by Raheman Vaiya (2017-).