Add a note describing the register naming convention in `raw_syscalls.rs` (for real this time).

core/platform/src/raw_syscalls.rs

diff --git a/core/platform/src/raw_syscalls.rs b/core/platform/src/raw_syscalls.rs
index b3cfba4..5f95d2b 100644
--- a/core/platform/src/raw_syscalls.rs
+++ b/core/platform/src/raw_syscalls.rs
@@ -10,7 +10,7 @@
 // RawSyscalls is designed to minimize the amount of handwritten assembly code
 // needed without generating unnecessary instructions. There are a few major
 // factors affecting its design:
-//     1. Most system calls only clobber r0-r4, while yield has a far longer
+//     1. Most system calls only clobber r0-r4 (*), while yield has a far longer
 //        clobber list. As such, yield must have its own assembly
 //        implementation.
 //     2. The compiler is unable to optimize away unused arguments. For example,
@@ -30,6 +30,10 @@
 //        ABI, so probably true for the Rust ABI), which are treated as
 //        clobbered regardless.
 //
+// (*) When this file refers to registers, it uses the same naming convention as
+// the Tock 2.0 syscalls TRD. Registers r0-r4 correspond to ARM registers r0-r4
+// and RISC-V registers a0-a4.
+//
 // Currently, yield takes exactly one argument, to specify what yield type to
 // do. Therefore we only need one raw yield call.
 //