php: Get rid of the phpXXbase attributes, update docs

Since the introduction of php.unwrapped there's no real need for the
phpXXbase attributes, so let's remove them to lessen potential
confusion and clutter. Also update the docs to make it clear how to
get hold of an unwrapped PHP if needed.

pkgs/development/interpreters/php/default.nix

@@ -302,5 +302,5 @@ let
   php72 = php72base.withExtensions defaultPhpExtensionsWithHash;
 
 in {
-  inherit php72base php73base php74base php72 php73 php74;
+  inherit php72 php73 php74;
 }