undocumented functions?

[Paul Zimmermann]

       Hi,

in the GMP 5.0.0 reference manual one can read:

      `mpn_divexact_by3', `mpn_modexact_1_odd' and the `mpn_redc_X'
   functions differ subtly in how they return r, leading to some negations
   in the above formula, but all are essentially the same.

However, while mpn_divexact_by3 is documented, mpn_modexact_1_odd and
mpn_redc_X are not.

Also there is a typo:

   hencethe use of `mpn_modexact_1_odd' by `mpn_gcd_1' and
   ^^^^^^^^

Paul