update documentation for simulate.tergm

[martinamorris]

as currently written, it appears to assume STERGM. for example:

https://github.com/statnet/tergm/blob/3080520c3078d903d2c8b2fef749afbf6109f556/R/simulate.tergm.R#L67-L71

if the monitor argument can only be used with Form() and Diss() operators, we should say this, otherwise this needs to be rewritten.

[krivit]

It can be used with all of them, I think.

[martinamorris]

And where do the monitored stats live in the output object?

I just ran the sim below, and I can see the stats.gen but not the monitored stats:

sep.sim <- simulate(sep.fit, nsim = 1, 
                    time.slices = 1000, 
                    monitor = "~edges",
                    stats = T)
names(attributes(sep.sim))
[1] "names"     "class"     "stats.gen" "coef"      "changes" 
``

[krivit]

Appended to stats, wherever they are.

[chad-klumb]

monitor should be either an ordinary formula (so, ~edges, not "~edges") or one of the character values "formation", "dissolution", or "all"

I think the character options should be removed (we can't make sense of them in the general non-separable case) but at the moment they are still there.