man: sort specifier list in systemd.unit(5) alphabetically

man: sort specifier list in systemd.unit(5) alphabetically

Usually, we order our settings in our unit files in a logical order,
grouping related settings together, and putting more relevant stuff
first, instead of following a strictly alphabetical order.

For specifiers I think it makes sense to follow an alphabetical order
however, since they literally are just characters, and hence I think the
concept of alphabetical ordering is much more commanding for them. Also,
since specifiers are usually not used in combination, but mostly used
indepdently of each other I think it's not that important to group
similar ones together.

No other changes except the reordering.