Move secp256k1_ecmult_odd_multiples_table_globalz_windowa and fix docs

[real-or-random]

secp256k1_ecmult_odd_multiples_table_globalz_windowa is in the wrong module. This function had been used within ecmult in the past but since 8c1c831, it is only used in ecmult_const. This also means its comment is wrong.

Originally posted by @real-or-random in #942 (comment)