improve docs about RemoveUnused

docs/rules/RemoveUnused.md

@@ -132,7 +132,22 @@ You may request more granular warnings to the compiler if you opt-out
 from some rewrites in the rule configuration.
 
 ```
-$ scala212 -Wunused:help
+$ scala212 -Ywarn-unused:help
+Enable or disable specific `unused' warnings
+  imports    Warn if an import selector is not referenced.
+  patvars    Warn if a variable bound in a pattern is unused.
+  privates   Warn if a private member is unused.
+  locals     Warn if a local definition is unused.
+  explicits  Warn if an explicit parameter is unused.
+  implicits  Warn if an implicit parameter is unused.
+  nowarn     Warn if a @nowarn annotation does not suppress any warnings.
+  params     Enable -Ywarn-unused:explicits,implicits.
+  linted     -Xlint:unused.
+Default: All choices are enabled by default.
+```
+
+```
+$ scala213 -Wunused:help
 Enable or disable specific `unused` warnings
   imports     Warn if an import selector is not referenced.
   patvars     Warn if a variable bound in a pattern is unused.

docs/users/installation.md

@@ -62,18 +62,18 @@ Next, if we run another rule like `RemoveUnused` then we get an error
 ```
 > myproject/scalafix RemoveUnused
 [error] (Compile / scalafix) scalafix.sbt.InvalidArgument: 2 errors
-[E1] The semanticdb-scalac compiler plugin is required to run semantic
+[E1] The scalac compiler should produce semanticdb files to run semantic
 rules like RemoveUnused ...
-[E2] The Scala compiler option "-Ywarn-unused" is required to use
-RemoveUnused ...
+[E2] A Scala compiler option is required to use RemoveUnused. To fix this
+problem, update your build to add -Ywarn-unused-import (with 2.12) ...
 ```
 
 The first error message means the
 [SemanticDB](https://scalameta.org/docs/semanticdb/guide.html) compiler plugin
 is not enabled for this project. The second error says `RemoveUnused` requires
 
 the Scala compiler option `-Ywarn-unused-import` (or `-Wunused:imports` in
-2.13.x). To fix both problems, add the following settings to `build.sbt`
+2.13.x or 3.x). To fix both problems, add the following settings to `build.sbt`
 
 ```diff
  /*
@@ -90,7 +90,12 @@ the Scala compiler option `-Ywarn-unused-import` (or `-Wunused:imports` in
  )
 
  lazy val myproject = project.settings(
-   scalacOptions += "-Ywarn-unused-import" // required by `RemoveUnused` rule
++  scalacOptions += {
++    if (scalaVersion.value.startsWith("2.12"))
++      "-Ywarn-unused-import"
++    else
++      "-Wunused:imports"
++  }
  )
 ```
 
@@ -104,7 +109,12 @@ the Scala compiler option `-Ywarn-unused-import` (or `-Wunused:imports` in
    scalaVersion := "@SCALA212@", // @SCALA213@, or 3.x
 +  semanticdbEnabled := true, // enable SemanticDB
 +  semanticdbVersion := scalafixSemanticdb.revision, // only required for Scala 2.x
-+  scalacOptions += "-Ywarn-unused-import" // Scala 2.x only, required by `RemoveUnused`
++  scalacOptions += {
++    if (scalaVersion.value.startsWith("2.12"))
++      "-Ywarn-unused-import"
++    else
++      "-Wunused:imports"
++  }
  )
 ```
 

scalafix-rules/src/main/scala/scalafix/internal/rule/OrganizeImports.scala

@@ -916,7 +916,7 @@ object OrganizeImports {
       Configured.error(
         "A Scala compiler option is required to use OrganizeImports with"
           + " \"OrganizeImports.removeUnused\" set to true. To fix this problem, update your"
-          + " build to add `-Ywarn-unused` (2.12) or `-Wunused:imports` (2.13 and 3.4+)."
+          + " build to add `-Ywarn-unused-import` (2.12) or `-Wunused:imports` (2.13 and 3.4+)."
       )
     else
       Configured.error(