swapinfo.8: Mention BLOCKSIZE in the ENVIRONMENT section

Copy the ENVIRONMENT section from df(1) to swapinfo(8), except
that the --si option is removed for swapinfo.

PR: 235225
Reported by: 0mp
MFC after: 3 days

(cherry picked from commit 39ad6e289f062fc8da239c0372f49b21f0911060)