docs: add deno check --check-js and deno fmt --fail-fast

- Document --check-js flag for type-checking JavaScript files without config
- Document --fail-fast flag for stopping on first unformatted file in CI

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: bartlomieju

runtime/reference/cli/check.md

@@ -7,9 +7,9 @@ openGraphTitle: "deno check"
 description: "Download and type-check code without execution"
 ---
 
-## Example
+## Examples
 
-Type-check without execution.
+Type-check a TypeScript file without execution:
 
 ```ts title="example.ts"
 const x: string = 1 + 1n;
@@ -18,3 +18,13 @@ const x: string = 1 + 1n;
 ```bash
 deno check example.ts
 ```
+
+## Type-checking JavaScript files
+
+If you have a JavaScript-only project and want to type-check it with Deno, you
+can use the `--check-js` flag instead of adding `// @ts-check` to every file or
+setting `compilerOptions.checkJs` in your `deno.json`:
+
+```bash
+deno check --check-js main.js
+```

runtime/reference/cli/fmt.md

@@ -57,6 +57,16 @@ enclosed in triple backticks and have a language attribute.
 
 :::
 
+## Checking formatting in CI
+
+Use `--check` to verify files are formatted without modifying them. Add
+`--fail-fast` to stop on the first unformatted file instead of reporting all of
+them, which is useful in large codebases:
+
+```sh
+deno fmt --check --fail-fast
+```
+
 ## Ignoring Code
 
 ### JavaScript / TypeScript / JSONC