update and improve description of deprecation policy

the old text said that if we (e.g.) deprecated something in 2.12.x, we
couldn't remove it until 2.14.0. that was never the intent, it isn't
what we have said elsewhere, and I don't think the community ever
understood that to be the policy, either.

Author: SethTisue

src/library/scala/deprecated.scala

@@ -39,20 +39,18 @@ import scala.annotation.meta._
  *  '''`@deprecated` in the Scala language and its standard library'''<br/>
  *
  *  A deprecated element of the Scala language or a definition in the Scala standard library will
- *  be preserved or at least another major version.
+ *  be preserved at least for the current major version.
  *
- *  This means that an element deprecated since 2.12 will be preserved in 2.13, but will very likely
- *  not be part of 2.14. Sometimes a deprecated element might be kept for more than a major
- *  release to ease migration and upgrades from older Scala versions.<br/>
- *  Developers should not rely on this.
+ *  This means that an element deprecated in some 2.12.x release will be preserved in
+ *  all 2.12.x releases, but may be removed in 2.13. (A deprecated element
+ *  might be kept longer to ease migration. Developers should not rely on this.)
  *
  *  '''Special deprecation policy for Scala 2.12'''<br>
- *  The Scala team has decided to enact a special deprecation policy for the 2.12 release:<br/>
+ *  The Scala team has decided to enact a special deprecation policy for Scala 2.12:<br/>
  *
- *  As an upgrade from Scala 2.11 to Scala 2.12 also requires upgrading from Java 6 to Java 8,
- *  no deprecated elements will be removed in this release to ease migration and upgrades
- *  from older Scala versions. This means that elements deprecated since 2.11 (or earlier)
- *  will not be removed in Scala 2.12.
+ *  As an upgrade from 2.11 to 2.12 also requires upgrading from Java 6 to Java 8,
+ *  deprecated elements will not normally be removed in this release, to ease migration
+ *  and cross-building.
  *
  *  @see    The official documentation on [[http://www.scala-lang.org/news/2.11.0/#binary-compatibility binary compatibility]].
  *  @param  message the message to print during compilation if the definition is accessed