man: clarify that Requires stop propagation only applies to explit requests

man: clarify that Requires stop propagation only applies to explit requests

Follow-up for e79eabdb1becc93cf4afc909aa18dc40c931eab5. There was an
apparent contradiction:

  man/systemd.unit says for Requires=:

  Besides, with or without specifying After=, this unit will be deactivated
  if one of the other units get deactivated.

  Also, some unit types may deactivate on their own (for example, a service
  process may decide to exit cleanly, or a device may be unplugged by the
  user), which is not propagated to units having a Requires= dependency.

Fixes #7870.