doc: package main can be directory with an index

This behavior dates back to 2011 but was not documented.

PR-URL: https://github.com/nodejs/node/pull/11581
Reviewed-By: Bradley Farias <bradley.meck@gmail.com>
Reviewed-By: James Snell <jasnell@gmail.com>

doc/api/modules.md

@@ -161,14 +161,18 @@ LOAD_AS_FILE(X)
 3. If X.json is a file, parse X.json to a JavaScript Object.  STOP
 4. If X.node is a file, load X.node as binary addon.  STOP
 
+LOAD_INDEX(X)
+1. If X/index.js is a file, load X/index.js as JavaScript text.  STOP
+2. If X/index.json is a file, parse X/index.json to a JavaScript object. STOP
+3. If X/index.node is a file, load X/index.node as binary addon.  STOP
+
 LOAD_AS_DIRECTORY(X)
 1. If X/package.json is a file,
    a. Parse X/package.json, and look for "main" field.
    b. let M = X + (json main field)
    c. LOAD_AS_FILE(M)
-2. If X/index.js is a file, load X/index.js as JavaScript text.  STOP
+   d. LOAD_INDEX(M)
-3. If X/index.json is a file, parse X/index.json to a JavaScript object. STOP
+2. LOAD_INDEX(X)
-4. If X/index.node is a file, load X/index.node as binary addon.  STOP
 
 LOAD_NODE_MODULES(X, START)
 1. let DIRS=NODE_MODULES_PATHS(START)