man: Describe when ExecStartFoo= commands are run in more detail

man: Describe when ExecStartFoo= commands are run in more detail

Previously it was just descibed that ExecStartPost= commands were
started "after" the ExecStart= command(s).
This hasn't specified after which event, which varies from after it has
been started, after it has exited, after it has sent READY=1 or after it
has taken the bus name, depending on Type=.

This now describes that it happens after the *service* has "started",
as defined by the Type=, and provides some clarification about precisely
when this is.

This may be unnecessary duplication, but it removes the ambiguity as to
whether RemainAfterExit=no means that ExecStartPost= shouldn't be
started because it means the service has stopped when the ExecStart=
command terminates, not "started".