overview: some rewordings

Author: andrewrk

content/en-US/learn/overview.smd

@@ -36,7 +36,7 @@ Zig has four [build modes](https://ziglang.org/documentation/master/#Build-Mode)
 
 | Parameter | [Debug](https://ziglang.org/documentation/master/#Debug) | [ReleaseSafe](https://ziglang.org/documentation/master/#ReleaseSafe) | [ReleaseFast](https://ziglang.org/documentation/master/#ReleaseFast) | [ReleaseSmall](https://ziglang.org/documentation/master/#ReleaseSmall) |
 |-----------|-------|-------------|-------------|--------------|
-Optimizations - improve speed, harm debugging, harm compile time | | -O3 | -O3| -Os |
+Optimizations - improve speed, harm debugging, harm compile time | | On | On | On |
 Runtime Safety Checks - harm speed, harm size, crash instead of undefined behavior | On | On | | |
 
 Here is what [Integer Overflow](https://ziglang.org/documentation/master/#Integer-Overflow) looks like at compile time, regardless of the build mode:
@@ -58,11 +58,11 @@ Zig uses [undefined behavior](https://ziglang.org/documentation/master/#Undefine
 
 Speaking of performance, Zig is faster than C.
 
-- The reference implementation uses LLVM as a backend for state of the art optimizations.
-- What other projects call "Link Time Optimization" Zig does automatically.
-- For native targets, advanced CPU features are enabled (-march=native), thanks to the fact that [Cross-compiling is a first-class use case](#cross-compiling-is-a-first-class-use-case).
-- Carefully chosen undefined behavior. For example, in Zig both signed and unsigned integers have undefined behavior on overflow, contrasted to only signed integers in C. This [facilitates optimizations that are not available in C](https://godbolt.org/z/n_nLEU).
+- All Zig code lives in one compilation unit, optimized together.
+- Carefully chosen illegal behavior. For example, in Zig both signed and unsigned integers have illegal behavior on overflow, contrasted to only signed integers in C. This [facilitates optimizations that are not available in C](https://godbolt.org/z/n_nLEU).
 - Zig directly exposes a [SIMD vector type](https://ziglang.org/documentation/master/#Vectors), making it easy to write portable vectorized code.
+- The standard library provides essential data structures such as hash maps and array lists, whereas in C it is tempting to use linked lists for simplicity.
+- Advanced CPU features are enabled by default, unless [cross-compiling](#cross-compiling-is-a-first-class-use-case).
 
 Please note that Zig is not a fully safe language. For those interested in following Zig's safety story, subscribe to these issues: