Small edits for lispref/os.texi

Small edits for lispref/os.texi

* doc/lispref/os.texi (Startup Summary): Copyedits.  Fix startup screen logic.
(Init File): Copyedits.
(Command-Line Arguments): Copyedits.  Do not mention argv alias.

* lisp/startup.el (normal-top-level, command-line, command-line-1):
Give them doc strings.