reword documentation on builtins

- add anchor to `builtins`
- add type information
- reword description of `builtins` to offer more information concisely

doc/manual/src/language/builtin-constants.md

@@ -1,20 +1,19 @@
 # Built-in Constants
 
-Here are the constants built into the Nix expression evaluator:
+These constants are built into the Nix language evaluator:
+
+- [`builtins`]{#builtins-builtins} (attribute set)
+
+  Contains all the [built-in functions](./builtins.md) and values, in order to avoid polluting the global scope.
+
+  Since built-in functions were added over time, [testing for attributes](./operators.md#has-attribute) in `builtins` can be used for graceful fallback on older Nix installations:
 
-- `builtins`\
-  The set `builtins` contains all the built-in functions and values.
-  You can use `builtins` to test for the availability of features in
-  the Nix installation, e.g.,
-  
   ```nix
   if builtins ? getEnv then builtins.getEnv "PATH" else ""
   ```
-  
-  This allows a Nix expression to fall back gracefully on older Nix
-  installations that don’t have the desired built-in function.
 
-- [`builtins.currentSystem`]{#builtins-currentSystem}\
+- [`builtins.currentSystem`]{#builtins-currentSystem} (string)
+
   The built-in value `currentSystem` evaluates to the Nix platform
   identifier for the Nix installation on which the expression is being
   evaluated, such as `"i686-linux"` or `"x86_64-darwin"`.