Document ":=" operator in the "model.syntax" topic

[tzoltak]

Operator ":=" is a part of the lavaan model syntax but it isn't documented in the "model.syntax" documentation topic, where all the other operators, as well as modifiers, are described. (In fact the only place I found information about ":=" is lavaan's webpage tutorial in the topic regarding mediation.) I think that it will be good to add description of this operator in this documentation topic.

[TDJorgensen]

The operators for (in)equality constraints are also not listed. I'll work on a quick PR.