Copyedits to FFI and unwinding section

These are minor changes that make the text flow better to me.

src/ffi.md

@@ -722,16 +722,19 @@ No `transmute` required!
 ## FFI and unwinding
 
 It’s important to be mindful of unwinding when working with FFI. Each
-non-`Rust` ABI comes in two variants, one with `-unwind` suffix and one without. If
+ABI comes in two variants, one with `-unwind` suffix and one without (except
-you expect Rust `panic`s or foreign (e.g. C++) exceptions to cross an FFI
+the `Rust` ABI, which always permits unwinding).
-boundary, that boundary must use the appropriate `-unwind` ABI string (note
-that compiling with `panic=abort` will still cause `panic!` to immediately
-abort the process, regardless of which ABI is specified by the function that
-`panic`s).
 
+If you expect Rust `panic`s or foreign (e.g. C++) exceptions to cross an FFI
+boundary, that boundary must use the appropriate `-unwind` ABI string.
 Conversely, if you do not expect unwinding to cross an ABI boundary, use one of
-the non-`unwind` ABI strings (other than `Rust`, which always permits
+the non-`unwind` ABI strings.
-unwinding). If an unwinding operation does encounter an ABI boundary that is
+
+> Note: Compiling with `panic=abort` will still cause `panic!` to immediately
+abort the process, regardless of which ABI is specified by the function that
+`panic`s.
+
+If an unwinding operation does encounter an ABI boundary that is
 not permitted to unwind, the behavior depends on the source of the unwinding
 (Rust `panic` or a foreign exception):