Strengthen wording on "C" ABI

When using the "C" ABI, do we "match" the default ABI chosen by the
dominant C compiler for the target or do we "attempt" to match it?

Tough question.  There are believed to be some exceptions in current
behavior.

However, in the Reference, we generally state things in terms of what
the consensus is on how they should be and then note the bugs, so
let's do that here.  We'll just say that we "match" it.

Author: traviscross

src/items/external-blocks.md

@@ -121,7 +121,7 @@ r[items.extern.abi.rust]
 * `unsafe extern "Rust"` --- The native calling convention for Rust functions and closures. This is the default when a function is declared without using [`extern fn`]. The Rust ABI offers no stability guarantees.
 
 r[items.extern.abi.c]
-* `unsafe extern "C"` --- The "C" ABI attempts to match the default ABI chosen by the dominant C compiler for the target.
+* `unsafe extern "C"` --- The "C" ABI matches the default ABI chosen by the dominant C compiler for the target.
 
 r[items.extern.abi.system]
 * `unsafe extern "system"` --- This is equivalent to `extern "C"` except on Windows x86_32 where it is equivalent to `"stdcall"`.