Improve sbt-plugin documentation

Author: jonas

sbt-scala-native-bindgen/src/main/scala/org/scalanative/bindgen/sbt/ScalaNativeBindgenPlugin.scala

@@ -28,13 +28,21 @@ import org.scalanative.bindgen.Bindgen
  *
  * Keys are defined in [[ScalaNativeBindgenPlugin.autoImport]].
  *
+ *  - `nativeBindgenPath`: Path to the `scala-native-bindgen` executable.
  *  - `nativeBindgenHeader`: The C header file to read.
  *
  *  - `nativeBindgenPackage`: Package of the enclosing object.
  *    No package by default.
  *
  *  - `name in nativeBindgen`: Name of the enclosing object.
  *
+ *  - `version in nativeBindgen`: Version of the `scala-native-bindgen`
+ *    to use when automatically downloading the executable.
+ *
+ *  - `nativeBindgenLink`: Name of library to be linked.
+ *
+ *  - `nativeBindgen`: Generate Scala Native bindings.
+ *
  * @example
  * {{{
  * nativeBindgenHeader in Compile := file("/usr/include/ctype.h")