The unified diff is not very helpful; I think somebody has to put the files side by side and merge.
The pydoc topics are built with a custom Sphinx builder implemented in tools/sphinxext/pyspecific.py -- but if we just want the vanilla text builder output it should be enough to add a Doc/Makefile target:
idledoc: BUILDER = text
idledoc: SOURCES = library/idle
idledoc: build
@echo "Build finished; now copying build/text/library/idle.txt to ../Lib/idlelib/help.txt."
@cp build/text/library/idle.txt ../Lib/idlelib/help.txt
and I add a step to PEP 101 to run this.