The WHERE clause introduced by 31f38174 was not described.  While on it,
split the grammar of \copy FROM and TO into two distinct parts for
clarity as they support different set of options.

Author: Vignesh C
Discussion: https://postgr.es/m/CALDaNm3zWr=OmxeNqOqfT=uZTSdam_j-gkX94CL8eTNfgUtf6A@mail.gmail.com
Backpatch-through: 12