Dev doc update.
* jk/diff-rendered-docs:
Revert "doc/Makefile: drop doc-diff worktree and temporary files on "make clean""
doc/Makefile: drop doc-diff worktree and temporary files on "make clean"
doc-diff: add --clean mode to remove temporary working gunk
doc-diff: fix non-portable 'man' invocation
doc-diff: always use oids inside worktree
SubmittingPatches: mention doc-diff