clarify recoverable vs non-recoverable panics

src/panic.md

@@ -16,13 +16,16 @@ over panic behavior:
 
 ## Unwinding
 
-Panicking may either be recoverable or non-recoverable, though its behavior
-must be uniform throughout program execution. A recoverable panic "unwinds"
-Rust frames, just as C++'s `throw` unwinds C++ frames. This means that as the
-panic traverses Rust frames, live objects in those frames that [implement
-`Drop`][destructors] will have their `drop` methods called. Thus, if panic
-recovery does occur (for instance at a thread boundary), the objects will have
-been "cleaned up" just as if they had gone out of scope normally.
+Panicking may either be recoverable or non-recoverable, though it can be
+configured (via `panic=abort`) to always be non-recoverable. (The converse is
+not true: `panic=unwind` does not guarantee that all panics are recoverable,
+only that panicking via the `panic!` macro and similar standard library
+mechanisms is recoverable.) A recoverable panic "unwinds" Rust frames, just as
+C++'s `throw` unwinds C++ frames. This means that as the panic traverses Rust
+frames, live objects in those frames that [implement `Drop`][destructors] will
+have their `drop` methods called. Thus, if panic recovery does occur (for
+instance at a thread boundary), the objects will have been "cleaned up" just as
+if they had gone out of scope normally.
 
 > **Note**: As long as this guarantee of resource-cleanup is preserved,
 > "unwinding" may be implemented without actually using the mechanism used by