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:

       7.12.10.3  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.

and

       [#1] implementation-defined behavior
       unspecified behavior where each implementation documents how
       the choice is made

So, how n is chosen should also be documented.