Preconditions of fe_equal_var are weaker than documented

[real-or-random]

From IRC:

<roconnor> The documentation for secp256k1_fe_equal_var suggest both inputs have to have magnitude 1; however only the first input needs to have magnitude 1. <roconnor> thus this call to fe_normalize_weak could be eliminated. <roconnor> But it is so very not critical, I'm not sure it is even worth PRing. <real_or_random> roconnor: same thing happens in secp256k1_gej_eq_x_var which is used in ECDSA verification. but ok, the speedup is still negligible <real_or_random> but I believe it's worth a PR (or an issue) because it would improve the internal docs

[roconnor-blockstream]

https://github.com/bitcoin-core/secp256k1/blob/1e78c18d5b80552d15aa2e2e58348fc1b89b1e3b/src/group_impl.h#L264 is the call to fe_normalize_weak I had in mind, but @real-or-random has another instance somewhere else.