Mergeinfo function combine_with_lastrange()

Paul,

Reviewing r39019 for back-port, I struggled to understand the doc string
of combine_with_lastrange(). It seemed to say bits and pieces about what
it did internally, without saying what its purpose or promises or
requirements were. I read through its implementation and its uses and
re-wrote the doc string as in the attached patch. I found that the
"lastrange" parameter was functionally redundant so I removed it, as it
just made the interface to the function more complex.

Please review the patch if you would.

Thanks.
- Julian

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