Update doc/specs/stdlib_constants.md

Co-authored-by: Jeremie Vandenplas <[email protected]>

Author: MilanSkocic

doc/specs/stdlib_constants.md

@@ -44,7 +44,7 @@ Experimental
 
 ### Description
 
-Convert a [[stdlib_codata_type(module):codata_constant_type(type)]] to a real. 
+Convert a [[stdlib_codata_type(module):codata_constant_type(type)]] to a `real` (at least `sp`, or `dp`) scalar. 
 **Warning**: Some constants cannot be converted to simple precision reals due to the values of the exponents.
 
 ### Syntax