Docs: Clarify definition of subject

doc/development/testing_guide/best_practices.md

@@ -448,7 +448,9 @@ so we need to set some guidelines for their use going forward:
 - `let!` variables should be used only in case if strict evaluation with defined
   order is required, otherwise `let` will suffice. Remember that `let` is lazy and won't
   be evaluated until it is referenced.
-- Use named `subject(:name)` over un-named `subject` in examples, as this gives the subject a contextual name.
+- Avoid referencing `subject` in examples. Use a named subject `subject(:name)`, or a `let` variable instead, so
+  the variable has a contextual name.
+- If the `subject` is never referenced inside examples, then it's acceptable to define the `subject` without a name.
 
 ### Common test setup