refactor: rename multipleMajorPrs -> separateMultipleMajor

lib/config/definitions.js

lib/config/migration.js

lib/manager/docker/package.js

lib/manager/npm/versions.js

test/config/__snapshots__/index.spec.js.snap

test/config/__snapshots__/migration.spec.js.snap

test/config/migration.spec.js

test/manager/docker/package.spec.js

test/manager/npm/versions.spec.js

website/docs/configuration-options.md