Auto merge of #9859 - koka831:no-run-side-effect, r=Manishearth

Avoid generating files via doctest

When we run `cargo test` in `clippy_lints` directory, it will generate [`foo.txt`](https://github.com/rust-lang/rust-clippy/blob/master/clippy_lints/foo.txt) in the directory.
In order to avoid that, this PR adds `no_run` to rustdoc which contains `File::create`.

changelog: none

clippy_lints/foo.txt

@@ -1 +0,0 @@
-Hello

clippy_lints/src/methods/mod.rs

@@ -3077,7 +3077,7 @@ declare_clippy_lint! {
     ///
     /// ### Example
     ///
-    /// ```rust
+    /// ```rust,no_run
     /// use std::fs::File;
     /// use std::io::{self, Write, Seek, SeekFrom};
     ///
@@ -3090,7 +3090,7 @@ declare_clippy_lint! {
     /// }
     /// ```
     /// Use instead:
-    /// ```rust
+    /// ```rust,no_run
     /// use std::fs::File;
     /// use std::io::{self, Write, Seek, SeekFrom};
     ///