Merge #15794: docs: Clarify PR guidelines w/re documentation

f4a230b docs: Clarify PR guidelines w/re documentation (Carl Dong)

Pull request description:

  PRs should change documentation accordingly when the behaviour of code changes.

ACKs for commit f4a230:
  practicalswift:
    ACK f4a230b
  fanquake:
    utACK f4a230b

Tree-SHA512: 6d9d65d7f0f9bc8f324ee16f03169df28fb512c58bb71093128cf16797b25533cdc992bc8757034a99d6fa6423a3129ca7cf2ab2da857535f409a683486fd4ab

Author: laanwj

CONTRIBUTING.md

@@ -220,6 +220,7 @@ In general, all pull requests must:
   - Not break the existing test suite;
   - Where bugs are fixed, where possible, there should be unit tests
     demonstrating the bug and also proving the fix. This helps prevent regression.
+  - Change relevant comments and documentation when behaviour of code changes.
 
 Patches that change Bitcoin consensus rules are considerably more involved than
 normal because they affect the entire ecosystem and so must be preceded by