doc: issue2551415 correct doc for change input->input_payload in 2.5 the rest interface changed a variable name from input to input_payload. An earlier commit changed the rest docs. This commit adds an item for it to the upgrading 2.4.0->2.5.0 section. Also cross reference added to the rest docs with the updated examples.

#!/usr/bin/env python

# Roundup requires Python 3.7+ as mentioned in doc\installation.txt
import sys

VERSION_NEEDED = (3, 7)

if sys.version_info < VERSION_NEEDED:
    print("Content-Type: text/plain\n")
    print("Roundup requires Python %s.%s or newer." % VERSION_NEEDED)
    sys.exit(0)