improve documentation in std

A lot of these "shorthand" doc comments were redundant, low quality
filler content. Better to let the actual modules speak for themselves
with top level doc comments rather than trying to document their
aliases.

lib/std/simd.zig

@@ -1,7 +1,9 @@
-//! This module provides functions for working conveniently with SIMD (Single Instruction; Multiple Data),
-//! which may offer a potential boost in performance on some targets by performing the same operations on
-//! multiple elements at once.
-//! Please be aware that some functions are known to not work on MIPS.
+//! SIMD (Single Instruction; Multiple Data) convenience functions.
+//!
+//! May offer a potential boost in performance on some targets by performing
+//! the same operations on multiple elements at once.
+//!
+//! Some functions are known to not work on MIPS.
 
 const std = @import("std");
 const builtin = @import("builtin");