reword ABI string category; make explicit that runtimes don't affect each other except at ABI boundary

Author: BatmanAoD

src/items/functions.md

@@ -214,9 +214,15 @@ behavior when unwinding out of a function.
 
 The table below indicates the behavior of an unwinding operation reaching each
 type of ABI boundary (function declaration or definition using the
-corresponding ABI string). Additionally, `C` in the ABI strings may be
-substituted with `stdcall` or any other ABI (other than `Rust`)
-supported by the language implementation.
+corresponding ABI string). Note that the Rust runtime is not affected by, and
+cannot have an effect on, any unwinding that occurs entirely within another
+language's runtime, that is, unwinds that are thrown and caught without
+reaching a Rust ABI boundary.
+
+The "unwinding" ABI category refers to `"Rust"` (the implicit ABI of Rust
+functions not marked `extern`), `"C-unwind"`, and any other ABI with `-unwind`
+in its name. The "non-unwinding" ABI category refers to all other ABI strings,
+including `"C"` and `"stdcall"`.
 
 Native unwinding is defined per-target. On targets that support throwing and
 catching C++ exceptions, it refers to the mechanism used to implement this
@@ -225,12 +231,12 @@ unwinding"][forced-unwinding]; `longjmp` on Windows and `pthread_exit` in
 `glibc` are implemented this way. Forced unwinding is explicitly excluded
 from the "Native unwind" column in the table.
 
-| panic runtime  | ABI          | `panic`-unwind                        | Native unwind (unforced) |
-| -------------- | ------------ | ------------------------------------- | ----------------------- |
-| `panic=unwind` | `"C-unwind"` | unwind                                | unwind                  |
-| `panic=unwind` | `"C"`        | abort                                 | [Undefined Behavior]    |
-| `panic=abort`  | `"C-unwind"` | `panic!` aborts (no unwinding occurs) | abort                   |
-| `panic=abort`  | `"C"`        | `panic!` aborts (no unwinding occurs) | [Undefined Behavior]    |
+| panic runtime  | ABI           | `panic`-unwind                        | Native unwind (unforced) |
+| -------------- | ------------  | ------------------------------------- | -----------------------  |
+| `panic=unwind` | unwinding     | unwind                                | unwind                   |
+| `panic=unwind` | non-unwinding | abort                                 | [Undefined Behavior]     |
+| `panic=abort`  | unwinding     | `panic!` aborts (no unwinding occurs) | abort                    |
+| `panic=abort`  | non-unwinding | `panic!` aborts (no unwinding occurs) | [Undefined Behavior]     |
 
 [panic-modes]: ../panic.md#panic-runtimes
 [forced-unwinding]: https://rust-lang.github.io/rfcs/2945-c-unwind-abi.html#forced-unwinding