[2.x] Document plugin cross building

src/reference/changes/migrating-from-sbt-1.x.md

@@ -23,14 +23,32 @@ In sbt 1.x bare settings were project settings that applied only to the root sub
 ```
 
 ```scala
-name := "root"
+name := "root" // every subprojects will be named root!
 publish / skip := true
 ```
 
 ### Migrating ThisBuild
 
 In sbt 2.x, bare settings settings should no longer be scoped to `ThisBuild`. One benefit of the new _common settings_ over `ThisBuild` is that it would act in a more predictable delegation. These settings are inserted between plugins settings and those defined in `settings(...)`, meaning they can be used to define settings like `Compile / scalacOptions`, which was not possible with `ThisBuild`.
 
+Cross building sbt plugins
+--------------------------
+
+In sbt 2.x, if you cross build an sbt plugin with Scala 3.x and 2.12.x, it will automatically cross build against sbt 1.x and sbt 2.x:
+
+```scala
+// using sbt 2.x
+lazy val plugin = (projectMatrix in file("plugin"))
+  .enablePlugins(SbtPlugin)
+  .settings(
+    name := "sbt-vimquit",
+  )
+  .jvmPlatform(scalaVersions = Seq("3.3.3", "2.12.20"))
+```
+
+If you use `projectMatrix`, make sure to move the plugin to a subdirectory like `plugin/`. Otherwise, the synthetic root project will also pick up the `src/`.
+Use sbt 1.10.2 or later to cross build from sbt 1.x side.
+
 Migrating to slash syntax
 -------------------------
 

src/reference/changes/sbt-2.0-change-summary.md

@@ -16,7 +16,8 @@ See also [Migrating from sbt 1.x](./migrating-from-sbt-1.x.md).
 - Bare settings are added to all subprojects, as opposed to just the root subproject, and thus replacing the role that `ThisBuild` has played. by [@eed3si9n][@eed3si9n] in [#6746][6746]
 - `test` task is changed to be incremental test that can cache test results. Use `testFull` for full test by [@eed3si9n][@eed3si9n] in [#7686][7686] 
 - sbt 2.x plugins are published with `_sbt2_3` suffix by [@eed3si9n][@eed3si9n] in [#7671][7671]
-- sbt 2.x adds `platform` setting so `ModuleID`'s `%%` operator can cross build on JVM as well as JS and Native, as opposed to `%%%` operator that was created in a plugin to workaround this issue. by [@eed3si9n][@eed3si9n] in [#6746][6746]
+- sbt 2.x adds `platform` setting so `ModuleID`'s `%%` operator can cross build on JVM as well as JS and Native, as opposed to `%%%` operator that was created in a plugin to workaround this issue, by [@eed3si9n][@eed3si9n] in [#6746][6746]
+- Dropped `useCoursier` setting so Coursier cannot be opted out, by [@eed3si9n][@eed3si9n] in [#7712][7712]
 
 ### Dropped dreprecations
 
@@ -41,5 +42,6 @@ See also:
   [7671]: https://github.com/sbt/sbt/pull/7671
   [7686]: https://github.com/sbt/sbt/pull/7686
   [7700]: https://github.com/sbt/sbt/pull/7700
+  [7712]: https://github.com/sbt/sbt/pull/7712
   [@eed3si9n]: https://github.com/eed3si9n
   [@adpi2]: https://github.com/adpi2