trim.8: minor update for manual page

Further explain that trim(8) is not for trimming free blocks
in populated file systems/ZFS pools, as people still take it wrong sometimes.

(cherry picked from commit dbc4a1c69191909a7210cad6da46b755557d0d34)
(cherry picked from commit b2f8d46d898fb50222aefca47d687a852c647b2f)