documentation: clarify how OSMContribution objects are created

[tyrasd]

The documentation already gives a good overview about the contribution view, including some of its quirks, but does currently not explain well how exactly these objects are created and exactly what we define as a modification. Possible improvements:

• mention the "minor version" concept/term used in some related work
• we squash all changes included in a single changeset (as long as there are no other changes from other changesets in the same time interval) to prevent "over-sampling" of artificial changes introduced by a "slow" upload.