Document the role of proposal dummy values

[marcoct]

This text implies that ProposalDummyValues are only used for distributions that have infinite (and discrete) support:

https://github.com/probcomp/PClean/blob/master/src/distributions/distributions.jl#L10-L14

But StringPrior has finite support (there is a maximum length), and it implements discrete_proposal_dummy_value:

https://github.com/probcomp/PClean/blob/master/src/distributions/string_prior.jl#L23-L25

[alex-lew]

Good question -- it's really for support that's too large to tractably enumerate, not just infinite support. It is used whenever preferred values are used (it's the "other" token from the PClean paper).