man: update systemctl man page for unit file commands, in particular "systemctl enable"

man: update systemctl man page for unit file commands, in particular "systemctl enable"

Clarify that "systemctl enable" can operate either on unit names or on unit
file paths (also, adjust the --help text to clarify this). Say that "systemctl
enable" on unit file paths also links the unit into the search path.

Many other fixes.

This should improve the documentation to avoid further confusion around #3706.