Add Known problems section

Author: Victor-N-Suadicani

clippy_lints/src/format_push_string.rs

@@ -14,6 +14,12 @@ declare_clippy_lint! {
     /// ### Why is this bad?
     /// Introduces an extra, avoidable heap allocation.
     ///
+    /// ### Known problems
+    /// `format!` returns a `String` but `write!` returns a `Result`.
+    /// Thus you are forced to ignore the `Err` variant to achieve the same API.
+    ///
+    /// While using `write!` in the suggested way should never fail, this isn't necessarily clear to the programmer.
+    ///
     /// ### Example
     /// ```rust
     /// let mut s = String::new();