The remquo function is not documented in manual/arith.texi; it should be added
to Section 20.8.4 "Remainder Functions".
Also note that the ISO C99 standard says:
18.104.22.168 The remquo functions
[#2] The remquo functions compute the same remainder as the
remainder functions. In the object pointed to by quo they store
a value whose sign is the sign of x/y and whose magnitude is
congruent modulo 2^n to the magnitude of the integral quotient
of x/y, where n is an implementation-defined integer greater
than or equal to 3.
[#1] implementation-defined behavior
unspecified behavior where each implementation documents how
the choice is made
So, how n is chosen should also be documented.