The probability of stumbling on a non-invertible b->A is negligible;
it's equivalent to accidentally factoring the RSA key. Relatedly,
document the slight caveat in BN_mod_inverse_blinded.

Change-Id: I308d17d12f5d6a12c444dda8c8fcc175ef2f5d45
Reviewed-on: https://boringssl-review.googlesource.com/26344
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>