add comments

Author: khoaHyh

docs/index.md

@@ -1061,7 +1061,7 @@ The option can be given multiple times. The option accepts a comma-delimited lis
 
 `--extension` now supports multipart extensions (e.g., `spec.js`), leading dots (`.js`) and combinations thereof (`.spec.js`);
 
-### `--file <file|directory|glob>`
+### `--file <file>`
 
 > _WARNING: `--file` is incompatible with [parallel mode](#parallel-tests)._
 

lib/cli/collect-files.js

@@ -59,6 +59,7 @@ module.exports = ({
   fileArgs.forEach(file => {
     const fileAbsolutePath = path.resolve(file);
     try {
+      // Used instead of fs.existsSync to ensure that file-ending less files are still resolved correctly
       require.resolve(fileAbsolutePath);
     } catch (err) {
       if (err.code === 'MODULE_NOT_FOUND') {