(help-setup-xref): Change all callers to

(help-setup-xref): Change all callers to
use (function args...) instead of (function . arg).  Doc fix.
(help-follow): Put (point) at front of elts of help-xref-stack.
(help-xref-stack): Elt format now (POSITION FUNCTION ARGS).
(help-xref-stack-item): Doc fix.
(help-xref-go-back): Assume new format for help-xref-stack.
(help-follow): Look for prop on previous char if next char has none.
Avoid error at beginning or end of buffer.
(describe-bindings): New optional arg BUFFER.
Use help-setup-xref.