[3.12] gh-97590: Update docs and tests for ftplib.FTP.voidcmd() (GH-96825) (GH-115601)

Since 2f3941d743481ac48628b8b2c075f2b82762050b this function returns the
response string, rather than nothing.
(cherry picked from commit e88ebc1c4028cf2f0db43659e513440257eaec01)

Co-authored-by: Matthew Hughes <34972397+matthewhughes934@users.noreply.github.com>