wpa_cli.8: describe better + tag spdx + linter nit

"how do I switch active wifi network on console"
"% apropos wifi"

MFC after: 3 days
Reviewed by: mhorne, imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1299

(cherry picked from commit 18a27dd64129f4b11ba458065bb5bb883f6b4c6e)