Update Default parameter values information (#3766)

The current text implies that it is possible to define default values for both types of positional parameters (required and optional), as it says: "Your function can use = to define default values for both named and positional parameters.". However, positional parameters can be of 2 types, required and optional, and only the optional ones allow the definition of a default value.

This change is intended to make it clear that only named and optional positional parameters are likely to have default values set.

Co-authored-by: Parker Lougheed <parlough@gmail.com>

Author: myronrezzo

src/_guides/language/language-tour.md

@@ -1398,8 +1398,8 @@ assert(say('Bob', 'Howdy', 'smoke signal') ==
 <a id="default-parameters"></a>
 #### Default parameter values
 
-Your function can use `=` to define default values for both named and positional
-parameters. The default values must be compile-time constants.
+Your function can use `=` to define default values for optional parameters,
+both named and positional. The default values must be compile-time constants.
 If no default value is provided, the default value is `null`.
 
 Here's an example of setting default values for named parameters: