Extend library commentary

Author: tarsius

emacsql.el

@@ -18,6 +18,11 @@
 ;; PostgreSQL and MySQL are also supported, but use of these connectors
 ;; is not recommended.
 
+;; Any readable lisp value can be stored as a value in EmacSQL,
+;; including numbers, strings, symbols, lists, vectors, and closures.
+;; EmacSQL has no concept of TEXT values; it's all just lisp objects.
+;; The lisp object `nil' corresponds 1:1 with NULL in the database.
+
 ;; See README.md for much more complete documentation.
 
 ;;; Code: