Documentation improvements around the db interface (#12362)

Added more details about the limits and reasoning behind the API.
Came about from this discussion on IRC: https://irclogs.nim-lang.org/04-10-2019.html#16:58:04

lib/impure/db_sqlite.nim

@@ -94,6 +94,18 @@
 ##
 ##    db.close()
 ##
+##
+## Note
+## ====
+## This module does not implement any ORM features such as mapping the types from the schema.
+## Instead, a ``seq[string]`` is returned for each row.
+##
+## The reasoning is as follows:
+## 1. it's close to what many DBs offer natively (char**)
+## 2. it hides the number of types that the DB supports
+## (int? int64? decimal up to 10 places? geo coords?)
+## 3. it's convenient when all you do is to forward the data to somewhere else (echo, log, put the data into a new query)
+##
 ## See also
 ## ========
 ##