add comments to date translation to clarify non-SQL origins (months(), days() on Postgres + Snowflake)

[fh-mthomson]

I found myself confused while trying to trace back date translations in dbplyr to their non-SQL origins, specifically base vs. lubridate functions:

• months() to lubridate, when the translations actually correspond to base::months()
• days() to base R (actually corresponds to lubridate::days()

I've proposed minor comments to hopefully prevent this confusion for future folks referencing these backend-specific translations for days() and months() (only available for Postgres and Snowflake - the latter was mirrored from the former).

cc: @fh-afrachioni