Python DataFrame documentation fixes

select empty should NOT be the same as select. make sure selectExpr is behaving the same.

join param documentation

link to source doesn't work in jekyll generated file

cross reference of columns (i.e. enabling linking)

show(): move df example before df.show()

move tests in SQLContext out of docstring otherwise doc is too long

Column.desc and .asc doesn't have any documentation

in documentation, sort functions.* (and add the functions defined in the dict automatically)

if possible, add index for all functions and classes

GroupedData doesn't have any description in the table of contents for pyspark.sql module