bazel: Add magic `roll` comment to MODULE.bazel

Signed-off-by: Chris Frantz <cfrantz@google.com>

MODULE.bazel

diff --git a/MODULE.bazel b/MODULE.bazel
index 332ce62..4b111c0 100644
--- a/MODULE.bazel
+++ b/MODULE.bazel
@@ -22,7 +22,7 @@
 
 git_override(
     module_name = "pigweed",
-    commit = "7882c537d21171b0e9641a132842a373382a78c3",
+    commit = "7882c537d21171b0e9641a132842a373382a78c3",  # roll:pigweed
     patch_args = ["-p1"],
     patches = [
         # Allow integration_tests to build for armv7-m / armv7e-m so the