[SPARK-11339][SPARKR] Document the list of functions in R base package that are masked by functions with same name in SparkR

Added tests for function that are reported as masked, to make sure the base:: or stats:: function can be called.

For those we can't call, added them to SparkR programming guide.

It would seem to me `table, sample, subset, filter, cov` not working are not actually expected - I investigated/experimented with them but couldn't get them to work. It looks like as they are defined in base or stats they are missing the S3 generic, eg.
```
> methods("transform")
[1] transform,ANY-method       transform.data.frame
[3] transform,DataFrame-method transform.default
see '?methods' for accessing help and source code
> methods("subset")
[1] subset.data.frame       subset,DataFrame-method subset.default
[4] subset.matrix
see '?methods' for accessing help and source code
Warning message:
In .S3methods(generic.function, class, parent.frame()) :
  function 'subset' appears not to be S3 generic; found functions that look like S3 methods
```
Any idea?

More information on masking:
http://www.ats.ucla.edu/stat/r/faq/referencing_objects.htm
http://www.sfu.ca/~sweldon/howTo/guide4.pdf

This is what the output doc looks like (minus css):
![image](https://cloud.githubusercontent.com/assets/8969467/11229714/2946e5de-8d4d-11e5-94b0-dda9696b6fdd.png)

Author: felixcheung <felixcheung_m@hotmail.com>

Closes #9785 from felixcheung/rmasked.

1 files changed, 36 insertions, 1 deletions

diff --git a/docs/sparkr.md b/docs/sparkr.md
index a744b76be7..cfb9b41350 100644
--- a/docs/sparkr.md
+++ b/docs/sparkr.md
@@ -286,7 +286,7 @@ head(teenagers)
 
 # Machine Learning
 
-SparkR allows the fitting of generalized linear models over DataFrames using the [glm()](api/R/glm.html) function. Under the hood, SparkR uses MLlib to train a model of the specified family. Currently the gaussian and binomial families are supported. We support a subset of the available R formula operators for model fitting, including '~', '.', ':', '+', and '-'. 
+SparkR allows the fitting of generalized linear models over DataFrames using the [glm()](api/R/glm.html) function. Under the hood, SparkR uses MLlib to train a model of the specified family. Currently the gaussian and binomial families are supported. We support a subset of the available R formula operators for model fitting, including '~', '.', ':', '+', and '-'.
 
 The [summary()](api/R/summary.html) function gives the summary of a model produced by [glm()](api/R/glm.html).
 
@@ -351,3 +351,38 @@ summary(model)
 ##Sepal_Width    0.404655
 {% endhighlight %}
 </div>
+
+# R Function Name Conflicts
+
+When loading and attaching a new package in R, it is possible to have a name [conflict](https://stat.ethz.ch/R-manual/R-devel/library/base/html/library.html), where a
+function is masking another function.
+
+The following functions are masked by the SparkR package:
+
+<table class="table">
+  <tr><th>Masked function</th><th>How to Access</th></tr>
+  <tr>
+    <td><code>cov</code> in <code>package:stats</code></td>
+    <td><code><pre>stats::cov(x, y = NULL, use = "everything",
+           method = c("pearson", "kendall", "spearman"))</pre></code></td>
+  </tr>
+  <tr>
+    <td><code>filter</code> in <code>package:stats</code></td>
+    <td><code><pre>stats::filter(x, filter, method = c("convolution", "recursive"),
+              sides = 2, circular = FALSE, init)</pre></code></td>
+  </tr>
+  <tr>
+    <td><code>sample</code> in <code>package:base</code></td>
+    <td><code>base::sample(x, size, replace = FALSE, prob = NULL)</code></td>
+  </tr>
+  <tr>
+    <td><code>table</code> in <code>package:base</code></td>
+    <td><code><pre>base::table(...,
+            exclude = if (useNA == "no") c(NA, NaN),
+            useNA = c("no", "ifany", "always"),
+            dnn = list.names(...), deparse.level = 1)</pre></code></td>
+  </tr>
+</table>
+
+You can inspect the search path in R with [`search()`](https://stat.ethz.ch/R-manual/R-devel/library/base/html/search.html)
+