Mention framework.exports in app_mode_common.h

Somewhat recently another place that needs to be updated if the
app shim version number changes was added, so make sure to mention
that new place to hopefully avoid confusion whenever we next
have to bump this version.

Change-Id: Iac508034dabf1046983f6ca668beab8ebcc21a42
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4890301
Commit-Queue: Marijn Kruisselbrink <mek@chromium.org>
Commit-Queue: Mark Mentovai <mark@chromium.org>
Reviewed-by: Mark Mentovai <mark@chromium.org>
Auto-Submit: Marijn Kruisselbrink <mek@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1202019}