"line comment" language reference text seems wrong

[kylegoetz]

The current Unison language reference says line comments begin with "--(any char sequence)" and can only otherwise contain whitespace.

Discussion on the Unison slack leads me to believe that line comments can indeed be preceded by anything. So it's valid to have

DEFAULT_AGE = 16 -- this is default age because it's the minimum legal driving age in the US

Language ref should be updated accordingly.