Edit the 4.0.0 changelog and migration guide against 3.x

[lehni]

Reviews the 4.0.0 release notes after four release candidates, so every line describes what 4.0.0 does compared to 3.9.1.

• Remove the rc-only comparisons, and fold their remaining content into the items they belong to
• Merge overlapping items: conflicting placements, relations of moved rows, the Take the keys that upsertGraph() writes from slots shared by all places of a row #427 key rules, no-op patches, the many-to-many limit() fixes and graph insert batching
• Group the breaking changes by area, with all insertGraph() / upsertGraph() items together, and the graph fixes after the others
• Correct the 3.x behaviour stated for a BelongsToOneRelation model related by its key alone (Relate a BelongsToOneRelation model that upsertGraph() gets by its key alone #473), and state the HasOne swap limit against 3.x
• Migration guide: add a keySide section for custom relations, the HasOne swap, SQLite insert batching and $afterFind of the returned models, add the missing TOC entry, and drop a sentence about reference errors after partial writes that no longer applies
• Guide: describe how upsertGraph() writes the models below rows that it relates from another owner, now that it loads and writes them with the rest of the graph