Doc: add pre-branch task to run src/tools/copyright.pl.

Doc: add pre-branch task to run src/tools/copyright.pl.

It's common for some files with last year's copyright date
to sneak into the tree between early January (when we normally run
copyright.pl) and feature freeze.  Immediately before branching
the new release is an ideal time to fix the stragglers, so add a
note about it to the RELEASE_CHANGES checklist.

Discussion: https://postgr.es/m/CALa6HA4_Wu7-2PV0xv-Q84cT8eG7rTx6bdjUV0Pc=McAwkNMfQ@mail.gmail.com