prettify top-level doc comment

Thanks, @kcbanner.

Author: Jan Philipp Hafer

lib/std/os/posix.zig

@@ -1,9 +1,10 @@
 //! This file contains posix abstractions and wrappers around
 //! OS-specific APIs for conforming platforms, whether libc is or is not linked.
-//! Purpose is to have error handling for APIs, which can not have comparable
-//! semantics on non-posix systems like Windows.
-//! Reasons are besides design failures (signaling, process management) more
-//! extensive execution semantics (permission system, memory management, ipc).
+//! The purpose of this file is to have error handling for APIs which
+//! don't support comparable semantics on non-posix systems like Windows.
+//! Reasons are:
+//!  - Design failures (signaling, process management)
+//!  - More extensive execution semantics (permission system, memory management, ipc)
 //! With above restrictions, the same goals as for os.zig apply.
 
 const std = @import("std");