Merge pull request #425 from rxin/scaladoc

API doc update & make Broadcast public

In #413 Broadcast was mistakenly made private[spark]. I changed it to public again. Also exposing id in public given the R frontend requires that.

Copied some of the documentation from the programming guide to API Doc for Broadcast and Accumulator.

This should be cherry picked into branch-0.9 as well for 0.9.0 release.

(cherry picked from commit 2ce23a55a3c4033873bb262919d89e5afabb9134)
Signed-off-by: Reynold Xin <rxin@apache.org>