PHB(special>go): Add instructions for overriding go.mod

The USES=go:modules system is smart and clever, but it does all its
evaluation of go.mod during fetch phase, which means that porters can't
patch it first (say, to alter dependencies or change the toolchain
version).

Instead, it's possible to override go.mod by specifying a new one as
a second distfile (and go.sum can be overridden in the same way). We've
supported that for a long time, but we've never documented it before.

Reviewed by: fuz, ziaee
Approved by: ziaee (doc)
Differential Revision: https://reviews.freebsd.org/D55580