Clarify need for borel distribution functions in the package

[jamesmbaazam]

My understanding is that these functions are not directly used in the package and are tangential in terms of scope. However, since the sampling process is derived from a branching process, it is convenient and can make sense to have them here.

If the decision is to keep them here, it would be good to call this out explicitly in the docs.

Originally posted by @Bisaloo in #122 (comment)