[coverage] Add comment about Siso starlark file in clang_code_coverage_wrapper.py

Unlike Ninja, Siso evaluates the clang code coverage wrapper logic
by using Starlark config. https://crrev.com/c/4659463

This is important for remote execution to not include the instrument
file to remote inputs.
Skipping the python wrapper script also improves build performance a lot.

When updating the Python script, please also update
the Siso's starlark file.

Bug: b/278225415
Change-Id: I2093219378827c53df180485665e7772c26f51ad
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4664322
Auto-Submit: Junji Watanabe <jwata@google.com>
Reviewed-by: Fumitoshi Ukai <ukai@google.com>
Commit-Queue: Prakhar Asthana <pasthana@google.com>
Reviewed-by: Prakhar Asthana <pasthana@google.com>
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1165973}