The 15 minute rule "thing" is in FM1, under Crew Qualification and Responsibility, Flight Crew Reporting and Release, Paragraph F. The first sentence:
"Pilots will be released from duty 15 minutes after block in of a flight (30 minutes if required to clear customs), unless required to reposition an aircraft, in which case the release time will be extended as necessary to complete the repositioning."
The pilot contract is not the only document.
That is the second item if you search the FM1 pdf for "15 min".
I agree that relying on "the 15 minute rule" as referenced by "the internet forum" is a great way to get yourself into trouble.
At best, anything you read on internet forums is a pointer for you to research yourself if it is important to you, including this post - you shouldn't just trust my "ctrl+f" abilities, I'm falible and I'm some entity on an internet forum. If this subject is important to you, then follow up on your own until you are comfortable enough with your own knowledge to ignore a CS call after your last leg.
Until then, if you want to go home - HI3 + HI6 (follow up on this yourself).
If you want to fly more, call CS and say "Can I fly more please?" (follow up on this yourself - you have duty limits).