Update docs/specs/stdlib_constants.md.

Author: MilanSkocic

doc/specs/stdlib_constants.md

@@ -6,8 +6,13 @@ title: constants
 
 ## Introduction
 
-`stdlib_constants` module provides mathematical and physical constants. 
-The latter are the last codata constants (2018) taken from [NIST](http://physics.nist.gov/constants.)
+
+The [[stdlib_constants]] module provides mathematical constants and the most common physical constants.
+
+The [[stdlib_codata]] module defines all codata constants as derived type [[codata_constant]].
+The lastest codata constants were released in 2018 by the [NIST](http://physics.nist.gov/constants.)
+The names are quite long and can be aliased with shorter names.
+
 
 ## Example