[SPARK-15412][PYSPARK][SPARKR][DOCS] Improve linear isotonic regression pydoc & doc build insturctions

## What changes were proposed in this pull request?

PySpark: Add links to the predictors from the models in regression.py, improve linear and isotonic pydoc in minor ways.
User guide / R: Switch the installed package list to be enough to build the R docs on a "fresh" install on ubuntu and add sudo to match the rest of the commands.
User Guide: Add a note about using gem2.0 for systems with both 1.9 and 2.0 (e.g. some ubuntu but maybe more).

## How was this patch tested?

built pydocs locally, tested new user build instructions

Author: Holden Karau <holden@us.ibm.com>

Closes #13199 from holdenk/SPARK-15412-improve-linear-isotonic-regression-pydoc.

1 files changed, 3 insertions, 1 deletions

diff --git a/docs/README.md b/docs/README.md
index bcea93e1f3..8b515e1873 100644
--- a/docs/README.md
+++ b/docs/README.md
@@ -20,8 +20,10 @@ installed. Also install the following libraries:
     $ sudo pip install Pygments
     # Following is needed only for generating API docs
     $ sudo pip install sphinx
-    $ Rscript -e 'install.packages(c("knitr", "devtools"), repos="http://cran.stat.ucla.edu/")'
+    $ sudo Rscript -e 'install.packages(c("knitr", "devtools", "roxygen2", "testthat"), repos="http://cran.stat.ucla.edu/")'
 ```
+(Note: If you are on a system with both Ruby 1.9 and Ruby 2.0 you may need to replace gem with gem2.0)
+
 ## Generating the Documentation HTML
 
 We include the Spark documentation as part of the source (as opposed to using a hosted wiki, such as