man: soft deprecate use of ";" for separating multiple command lines in ExecStart=

man: soft deprecate use of ";" for separating multiple command lines in ExecStart=

So far we supported this syntax:

    ExecStart=foo ; bar

as equivalent to:

    ExecStart=foo
    ExecStart=bar

With this change we'll "soft" deprecate the first syntax. i.e. it's
still supported in code, but not documented anymore.

The concept was originally added to make things easier for 3rd party
.ini readers, as it allowed writing unit files with a .ini framework
that doesn't allow multiple assignments for the same key. But frankly,
this is kinda pointless, as so many other of our knobs require the
double assignment.

Hence, let's just stop advertising the concept, let's simplify the docs,
by removing one entirely redundant feature from it.

Replaces: #34570
(cherry picked from commit 225f18b9a9d39331ea862478ab2ff893678e249d)