man: don't document systemctl --failed

This effectively reverts 599b6322f19ec619ddc294d0e7880b669040cf66, which
in turn partially reverted 4dc5b821ae737914499119e29811fc3346e3d97c.

The --failed switch is not documented on purpose, since it is redundant
due to --state=failed, which it predates. Due to that it's not
documented in --help either.

We generally try to avoid redundant interfaces, but if we need to keep
them for compatibility we do so, however remove them from documentation
to ensure they are not used in future.

The man page is now changed to include a comment about the fact that
--failed is not documented on purpose. Also, explicitly mention
--state=failed as example for --state.