Update documentation for the interface to_real.

Author: MilanSkocic

doc/specs/stdlib_constants.md

@@ -32,11 +32,39 @@ procedures.
 * 2 type-bound procedures:
 
     * `print` for print the values of the constant members.
-    * `to_real` for converting the value or the uncertainty to the desired precision.
+    * `to_real` for getting the value or the uncertainty to the desired precision.
 
-A module level interface [[stdlib_codata_type(module):to_real(interface)]] is available for converting the constant value
-or uncertainty.
+A module level interface [[stdlib_codata_type(module):to_real(interface)]] is 
+available for getting the constant value or uncertainty of a constant. 
 
+## `to_real` - Get the constant value or uncertainty.
+
+### Status
+
+Experimental
+
+### Description
+
+Convert a [[stdlib_codata_type(module):stdlib_codata_type(type)]] to a real. 
+**Warning**: Some constants cannot be converted to simple precision reals due to the values of the exponents.
+
+### Syntax
+
+`r = ` [[stdlib_codata_type(module):to_real(interface)]] `(c, mold [, uncertainty])`
+
+### Arguments
+
+`c`: argument has `intent(in) ` and shall be of type [[stdlib_codata_type(module):stdlib_codata_type(type)]].
+
+`mold`: argument has `intent(in)` and shall be of `real` type. 
+**Note**: The type of the `mold` argument defines the type of the result.
+
+`uncertainty` (optional): argument has `intent(in)` and shall be of `logical` type. 
+It specifies if the uncertainty needs to be returned instead of the value. Default to `.false.`.
+
+### Return value
+
+Returns a scalar of `real` type which is either the value or the uncertainty of a codata constant.
 
 ## Example
 

src/stdlib_codata_type.fypp

@@ -51,7 +51,7 @@ elemental pure real(${k}$) function to_real_${k}$(self, mold, uncertainty) resul
 
     class(codata_constant_type), intent(in) :: self !! Codata constant
     real(${k}$), intent(in) :: mold !! dummy argument to disambiguate at compile time the generic interface
-    logical, intent(in), optional :: uncertainty !! Set to true if the uncertainty is required
+    logical, intent(in), optional :: uncertainty !! Set to true if the uncertainty is required. Default to .false..
         !! 
     logical :: u