lib: document gc problem related to seq-begin.

If seq-begin is used on an object that supports the iter-begin
method, there is a gc problem. This does not seem worth
fixing for the following reasons.

1. The seq-begin function is marked obsolescent. I removed
   its one and only internal use in the previous commit, so
   it won't be called unless application code uses it.

2. Objects supporting the iter-begin function are clearly
   developed as part of the new iteration protocol. It makes
   no sense to be newly developing such an object, along with
   new code which applies seq-begin to it.  There is likely
   zero code in the wild which uses either of these
   mechanisms.

* lib.c (seq_iter_get_oop, seq_iter_peek_oop,
seq_iter_get_fast_oop, seq_iter_peek_fast_oop): Add comments
documenting the issue.

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