cmd/compile: update ssa html help text

Update the description of the conditions under which highlighting might
be misleading.

Fixes #16754

Change-Id: I3078a09e0b9a76d12078352e15a3f26ba3f1bbee
Reviewed-on: https://go-review.googlesource.com/30818Reviewed-by: David Chase <drchase@google.com>

src/cmd/compile/internal/ssa/html.go

@@ -274,9 +274,10 @@ function toggle_visibility(id) {
 <div id="help">
 
 <p>
-Click on a value or block to toggle highlighting of that value/block and its uses.
-Values and blocks are highlighted by ID, which may vary across passes.
-(TODO: Fix this.)
+Click on a value or block to toggle highlighting of that value/block
+and its uses.  (Values and blocks are highlighted by ID, and IDs of
+dead items may be reused, so not all highlights necessarily correspond
+to the clicked item.)
 </p>
 
 <p>