Cohort subset documentation

[anthonysena]

Noting some areas of improvement in the documentation for the cohort subset functionality:

• Documentation should make it clear how a cohort subset operator is related to a definition. I found this a little confusing in the vignette.
• Our function definitions should also delineate between operator functions and definition functions
• We should expand the existing vignette or create another vignette that showcases each operator in detail. In particular, the cohort subset operator is very powerful but it is hard to know how to build the proper time windows to achieve the subset(s) desired as evident in Unable to subset cohort based on 'during target cohort' #175.

Originally posted by @anthonysena in #197 (review)