Introduce for3Use2_13 and for3_13Use3

src/reference/02-DetailTopics/01-Using-sbt/03-Cross-Build.md

@@ -285,6 +285,29 @@ depending on the value of `crossPaths`.
 Because (unlike Scala library) Scala compiler is not forward compatible among
 the patch releases, compiler plugins should use `CrossVersion.full`.
 
+#### Scala 3 specific cross-versions 
+
+In a Scala 3 project you can use Scala 2.13 libraries:
+
+```scala
+("a" % "b" % "1.0") cross CrossVersion.for3Use2_13
+```
+
+This is equivalent to using `%%` except it resolves the `_2.13` variant 
+of the library  when `scalaVersion` is 3.x.y.
+
+Conversely we have `CrossVersion.for2_13Use3` to use the `_3` variant of the
+library when `scalaVersion` is 2.13.x:
+
+```scala
+("a" % "b" % "1.0") cross CrossVersion.for2_13Use3
+```
+
+**Warning for library authors:** It is generally not safe to publish
+a Scala 3 library that depends on a Scala 2.13 library or vice-versa.
+The reason is to prevent your end users from having two versions `x_2.13`
+and `x_3` of the same x library in their classpath.
+
 #### More about using cross-built libraries
 
 You can have fine-grained control over the behavior for different Scala versions