Tweak common tool docs on type implement trait check

Common tool documentation uses `match_trait_method` which is deprecated.
Additionally, adds information on why `is_trait_method` is not the
preferred way.

Author: dswij

doc/common_tools_writing_lints.md

@@ -78,14 +78,15 @@ impl LateLintPass<'_> for MyStructLint {
 There are two ways to do this, depending if the target trait is part of lang items.
 
 ```rust
-use clippy_utils::{implements_trait, match_trait_method, paths};
+use clippy_utils::{implements_trait, is_trait_method};
+use rustc_span::symbol::sym;
 
 impl LateLintPass<'_> for MyStructLint {
     fn check_expr(&mut self, cx: &LateContext<'_>, expr: &Expr<'_>) {
         // 1. Using expression and Clippy's convenient method
-        // we use `match_trait_method` function from Clippy's toolbox
-        if match_trait_method(cx, expr, &paths::INTO) {
-            // `expr` implements `Into` trait
+        // we use `is_trait_method` function from Clippy's toolbox
+        if is_trait_method(cx, expr, sym::Iterator) {
+            // method call in `expr` belongs to `Iterator` trait
         }
 
         // 2. Using type context `TyCtxt`
@@ -101,9 +102,7 @@ impl LateLintPass<'_> for MyStructLint {
 }
 ```
 
-> Prefer using lang items, if the target trait is available there.
-
-A list of defined paths for Clippy can be found in [paths.rs][paths]
+> Prefer using lang items, if the target trait is available there. `is_trait_method` checks for method call and can introduce unwanted behaviours.
 
 We access lang items through the type context `tcx`. `tcx` is of type [`TyCtxt`][TyCtxt] and is defined in the `rustc_middle` crate.