Expand on misrefactored_assign_op known problems

Author: dwijnand

clippy_lints/src/assign_ops.rs

@@ -51,8 +51,10 @@ declare_clippy_lint! {
 /// **Why is this bad?** Most likely these are bugs where one meant to write `a
 /// op= b`.
 ///
-/// **Known problems:** Someone might actually mean `a op= a op b`, but that
-/// should rather be written as `a = (2 * a) op b` where applicable.
+/// **Known problems:** Clippy cannot know for sure if `a op= a op b` should have
+/// been `a = a op a op b` or `a = a op b`/`a op= b`. Therefore it suggests both.
+/// If `a op= a op b` is really the correct behaviour then
+/// rewrite it as `a = (2 * a) op b` as it's less confusing.
 ///
 /// **Example:**
 /// ```rust