Add a code comment explaining the riscv64 TODOs in libm/Android.bp.

Test: treehugger
Change-Id: Ia52217533328321375bc0cd132b8571c9b9874f5
diff --git a/libm/Android.bp b/libm/Android.bp
index 4d3890d..2748871 100644
--- a/libm/Android.bp
+++ b/libm/Android.bp
@@ -346,6 +346,7 @@
             ],
 
             exclude_srcs: [
+                // TODO: do the rest when our clang has https://reviews.llvm.org/D136508.
                 // TODO: "upstream-freebsd/lib/msun/src/s_ceil.c",
                 // TODO: "upstream-freebsd/lib/msun/src/s_ceilf.c",
                 "upstream-freebsd/lib/msun/src/s_copysign.c",