doc: clarify required flag for extensionless esm

Fixes: https://github.com/nodejs/node/issues/30655

PR-URL: https://github.com/nodejs/node/pull/30657
Reviewed-By: Yorkie Liu <yorkiefixer@gmail.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>

doc/api/esm.md

@@ -84,7 +84,8 @@ If the nearest parent `package.json` lacks a `"type"` field, or contains
 `"type": "commonjs"`, extensionless and `.js` files are treated as CommonJS.
 If the volume root is reached and no `package.json` is found,
 Node.js defers to the default, a `package.json` with no `"type"`
-field.
+field. "Extensionless" refers to file paths which do not contain
+an extension as opposed to optionally dropping a file extension in a specifier.
 
 `import` statements of `.js` and extensionless files are treated as ES modules
 if the nearest parent `package.json` contains `"type": "module"`.