docs: document design

Author: bcoe

README.md

@@ -195,7 +195,8 @@ const { flags, values, positionals } = parseArgs(argv, options);
 - Does the API specify whether a `--` was present/relevant?
   - no
 - Is `-foo` the same as `--foo`?
-  - no, `-foo` is a short option or options (WIP: https://github.com/pkgjs/parseargs/issues/2)
+  - no, `-foo` is a short option or options, with expansion logic that follows the
+    [Utility Syntax Guidelines in POSIX.1-2017](https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap12.html). `--foo bar`, `withValue: ['foo']`, expands to `-f`, `-o`, `-o bar`.
 - Is `---foo` the same as `--foo`?
   - no 
   - the first flag would be parsed as `'-foo'`