docs: fixup describing "main" as package entry point

Author: GeoffreyBooth

doc/api/esm.md

@@ -151,8 +151,9 @@ package scope:
 
 ## Package Entry Points
 
-When a `package.json` contains `"type": "module"`, its `"main"` field defines
-the ES module entry point for the package.
+The `package.json` `"main"` field defines the entry point for a package,
+whether the package is included into CommonJS via `require` or into an ES
+module via `import`.
 
 <!-- eslint-skip -->
 ```js
@@ -177,6 +178,9 @@ CommonJS.
 As with `import` statements, for ES module usage the value of `"main"` must be
 a full path including extension: `"./index.mjs"`, not `"./index"`.
 
+If the `package.json` `"type"` field is omitted, a `.js` file in `"main"` will
+be interpreted as CommonJS.
+
 > Currently a package can define _either_ a CommonJS entry point **or** an ES
 > module entry point; there is no way to specify separate entry points for
 > CommonJS and ES module usage. This means that a package entry point can be