doc: update car, cdr, rplaca, rplacd.

* txr.1: Update the documentation to reflect the current
handling of objects, eliminating some contradictions
in the process, like text which says that rplacd is
erroneous on anything but conses and lazy conses, followed by
text which states other kinds of objects are allowed.
The semantics of car and cdr forms as places is described
strictly in terms of rplaca and rplacd, so the mechanism is
then described in one place.

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