make VAR+=VAL

[??????]

Hi Guys,


I saw GNU make (I am using 3.8.1) allows the usage of 'make VAR+=VAL' command 
line, but it does not append VAL to VAR and instead it overrides VAR to VAL. 


Is this expected behavior? I did not find any doc explaining the usage of 
VAR+=VAL on command line.


Thanks,
Justin