"iscachable" functions

For the "iscachable" parameter of CREATE FUNCTION, the 7.2.x
documentation states:
    "Iscachable indicates that the function always returns the same result
when given the same argument values (i.e., it does not do database
lookups or otherwise use information not directly present in its
parameter list). The optimizer uses iscachable to know whether it is
safe to pre-evaluate a call of the function."

But where is this cache?  Is it per backend connection, or is it shared
among all backends?  Also, is there a way to invalidate this cache or
otherwise expire the values?

thanks!

eric