Missing documentation regarding initialization with {} in C?

[tsun@cock.li]

There was recently a discussion with confusion regarding initialization 
with {} instead of {0} when initializing an empty array or struct. The 
former is not valid in C, but GCC permits it anyway even though it's not 
explained anywhere in the specs:
char something[20] = {};

Is it missing from the specs or did I just fail to locate it? I checked 
all the C extension pages but none of them had it.