[SPARK-19126][DOCS] Update Join Documentation Across Languages

## What changes were proposed in this pull request?

- [X] Make sure all join types are clearly mentioned
- [X] Make join labeling/style consistent
- [X] Make join label ordering docs the same
- [X] Improve join documentation according to above for Scala
- [X] Improve join documentation according to above for Python
- [X] Improve join documentation according to above for R

## How was this patch tested?
No tests b/c docs.

Please review http://spark.apache.org/contributing.html

 before opening a pull request.

Author: anabranch <wac.chambers@gmail.com>

Closes #16504 from anabranch/SPARK-19126.

(cherry picked from commit 19d9d4c8)
Signed-off-by: Felix Cheung <felixcheung@apache.org>