Add clippy configuration section to the manual

Author: lnicola

docs/user/manual.adoc

@@ -816,6 +816,10 @@ include::./generated_assists.adoc[]
 While most errors and warnings provided by rust-analyzer come from the `cargo check` integration, there's a growing number of diagnostics implemented using rust-analyzer's own analysis.
 Some of these diagnostics don't respect `\#[allow]` or `\#[deny]` attributes yet, but can be turned off using the `rust-analyzer.diagnostics.enable`, `rust-analyzer.diagnostics.experimental.enable` or `rust-analyzer.diagnostics.disabled` settings.
 
+=== Clippy
+
+To run `cargo clippy` instead of `cargo check`, you can set `"rust-analyzer.check.command": "clippy"`.
+
 include::./generated_diagnostic.adoc[]
 
 == Editor Features