man: systemd.exec - explicit Environment assignment

Hi all,

while working on another bug, I discovered the "strange" way systemd is
parsing Environment= in .service and thought it was worth documenting
(because I don't expect people to find this syntax by themselves unless
they read the parsing code ;)

Be more verbose about using space in Environment field and not
using value of other variables

Fixes https://bugzilla.redhat.com/show_bug.cgi?id=840260

[zj: expand and reformat the example a bit]