Clarify about Windows functions with explicit ABI

Some Windows API functions have an explicitly defined ABI.  We were
trying to suggest this with wording that was a bit subtle; let's spell
it out explicitly.

Author: traviscross

src/items/external-blocks.md

@@ -127,7 +127,7 @@ 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"`.
 
   > [!NOTE]
-  > As the correct underlying ABI on Windows is target-specific, it's best to use `extern "system"` when attempting to link Windows API functions that don't otherwise use a different ABI.
+  > As the correct underlying ABI on Windows is target-specific, it's best to use `extern "system"` when attempting to link Windows API functions that don't use an explicitly defined ABI.
 
 r[items.extern.abi.unwind]
 * `extern "C-unwind"` and `extern "system-unwind"` --- Identical to `"C"` and `"system"`, respectively, but with [different behavior][unwind-behavior] when the callee unwinds (by panicking or throwing a C++ style exception).