Move primop docs inline

This makes them available to 'nix repl'.

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

@@ -0,0 +1,20 @@
+# Built-in Constants
+
+Here are the constants built into the Nix expression evaluator:
+
+  - `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`  
+    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"`.