math/: Improve some commentary in the binary-field arithmetic.

math/: Improve some commentary in the binary-field arithmetic.

  * Explain why `gfreduce_trace' can safely return its answer as an int.

  * Explain how `gfreduce_quadsolve' actually works.  Also explicitly
    guarantee that its result is deterministic.

  * Explain how the `find' method works in `ec-bin.c'.

There's a little fiddling with braces to fit the new commentary in, but
no significant code change.