RepaintManager setDoubleBufferingEnabled spec is vague

Currently the RepaintManager supports a doubleBufferingEnabled property
which can be set to true/false by programs to control whether or not
Swing implements double-buffering for painting components.

Some platforms (Mac OS X) have native support for double-buffering and
hence do not want/need Swing's double-buffering. Programs should
generally not turn on Swing double-buffering under these conditions
so the spec should be modified to emphasize this.

Also the setting of the default should read a system property to
determine whether native DB support exists to correctly decide
whether Swing DB is on or off.