[SPARK-5994] [SQL] 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.*)

Author: Davies Liu <davies@databricks.com>

Closes #4756 from davies/df_docs and squashes the following commits:

f30502c [Davies Liu] fix doc
32f0d46 [Davies Liu] fix DataFrame docs