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