doc(Algebra/Group/Defs): add note on names SMul, VAdd; typo fix (#…

…9776)

Edited module docstring:
* Fixed recent accidental damage
* Added a note about the motivation for the names `SMul` and `VAdd`
[Zulip discussion](https://leanprover.zulipchat.com/#narrow/stream/113489-new-members/topic/.60SMul.60.20and.20.60VAdd.60)


Co-authored-by: Richard Copley <rcopley@gmail.com>

Mathlib/Algebra/Group/Defs.lean

@@ -28,12 +28,15 @@ The file does not contain any lemmas except for
 
 For basic lemmas about these classes see `Algebra.Group.Basic`.
 
-We also introduce notation classes `lul` and `VAdd` for multiplicative and additive
+We also introduce notation classes `SMul` and `VAdd` for multiplicative and additive
 actions and register the following instances:
 
 - `Pow M ℕ`, for monoids `M`, and `Pow G ℤ` for groups `G`;
 - `SMul ℕ M` for additive monoids `M`, and `SMul ℤ G` for additive groups `G`.
 
+`SMul` is typically, but not exclusively, used for scalar multiplication-like operators.
+See the module `Algebra.AddTorsor` for a motivating example for the name `VAdd` (vector addition)`.
+
 ## Notation
 
 - `+`, `-`, `*`, `/`, `^` : the usual arithmetic operations; the underlying functions are