Small clarification to edition compatibility

The old text ("The Rust compiler can link crates
of any editions together") made it sound like old
compilers would somehow magically be able to support
language features from newer editions.

The goal of this new text is to make it more clear
that a given compiler release will only be able to
work with the specific editions that it supports.

Author: eminence

src/editions/index.md

@@ -27,9 +27,11 @@ This serves different purposes for different people:
 When a new edition becomes available in the compiler, crates must explicitly opt
 in to it to take full advantage. This opt in enables editions to contain
 incompatible changes, like adding a new keyword that might conflict with
-identifiers in code, or turning warnings into errors. The Rust compiler can link
-crates of any editions together. Edition changes only affect the way the
-compiler initially parses the code. Therefore, if you're using Rust 2015, and
+identifiers in code, or turning warnings into errors. A Rust compiler will
+support all editions that existed prior to the compiler's release, and can link
+crates of any supported editions together.
+Edition changes only affect the way the compiler initially parses the code.
+Therefore, if you're using Rust 2015, and
 one of your dependencies uses Rust 2018, it all works just fine. The opposite
 situation works as well.