[Format] Make requirement around dense union offsets less ambiguous

[asfimport]

Currently, the spec states that dense union offsets for each child array must be "in order / increasing". There is an ambiguity: should they be strictly increasing, or are equal values supported?

The C++ implementation currently considers that equal offsets are acceptable.

 

Reporter: Antoine Pitrou / @pitrou

_{Note: This issue was originally created as ARROW-13479. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
cc @bkietz @wesm  @emkornfield

[asfimport]

Micah Kornfield / @emkornfield:
Hmm, I would have thought strictly, but I don't know the implications of changing it at this point.

[asfimport]

Antoine Pitrou / @pitrou:
Same opinion. We should probably be conservative. @wesm What do you say?