Update notes about static linking and MUSL

People often use the edition guide as general documentation of
capabilities, not just for Rust 2018. There is extensive information
on the web suggesting that Rust *only* supports static linking with
MUSL. Add a note documenting current capabilities, so that people know
they have a choice of libc implementations even if they need static
linking.

Author: joshtriplett

src/SUMMARY.md

@@ -84,6 +84,7 @@
       - [Global allocators](rust-2018/platform-and-target-support/global-allocators.md)
       - [MSVC toolchain support](rust-2018/platform-and-target-support/msvc-toolchain-support.md)
       - [MUSL support for fully static binaries](rust-2018/platform-and-target-support/musl-support-for-fully-static-binaries.md)
+        - Note: current Rust (as of 1.48) supports static binaries with either glibc or MUSL, so you can select whichever libc you need or prefer.
       - [`cdylib` crates for C interoperability](rust-2018/platform-and-target-support/cdylib-crates-for-c-interoperability.md)
 
 ## The Next Edition

src/rust-2018/platform-and-target-support/musl-support-for-fully-static-binaries.md

@@ -2,6 +2,9 @@
 
 ![Minimum Rust version: 1.1](https://img.shields.io/badge/Minimum%20Rust%20Version-1.1-brightgreen.svg)
 
+(Note: current Rust (as of 1.48) supports static linking with either glibc or
+MUSL, so you can select whichever libc you need or prefer.)
+
 By default, Rust will statically link all Rust code. However, if you use the
 standard library, it will dynamically link to the system's `libc`
 implementation.