Auto merge of rust-lang#6876 - giraffate:improve_doc_on_map_flatten, r=flip1995

Improve doc on `map_flatten`

Fix rust-lang/rust-clippy#6870. It's because this doc lacks the description that this lint is also used for `Option`.

changelog: none

Author: bors

clippy_lints/src/methods/mod.rs

@@ -403,7 +403,7 @@ declare_clippy_lint! {
 }
 
 declare_clippy_lint! {
-    /// **What it does:** Checks for usage of `_.map(_).flatten(_)`,
+    /// **What it does:** Checks for usage of `_.map(_).flatten(_)` on `Iterator` and `Option`
     ///
     /// **Why is this bad?** Readability, this can be written more concisely as
     /// `_.flat_map(_)`