Spaces inserted by the C preprocessor

Suppose we are given this input C code:

#define Y 20
#define A(x) (10+x+Y)

A(A(40))

gcc -E outputs like that (10+(10+40 +20)+20).

gcc -E -traditional-cpp outputs like that (10+(10+40+20)+20).

Why the default cpp inserts the space after 40 ?

Where can I find the most detailed specification of the cpp that covers that logic ?