Better Doc comments for db.pg

[jacksonmowry]

Would it be possible to update the documentation for db.pg (specifically exec_param, exec_param2, and exec_many) to include how to make a parameterized query string. The syntax ($1) ($2) ($n) is used to specify where arguments get placed in the query. I think it would be helpful to add this to the doc comment for these functions, if not something in the readme.