[SPARK-3430] [PySpark] [Doc] generate PySpark API docs using Sphinx

Using Sphinx to generate API docs for PySpark.

requirement: Sphinx

```
$ cd python/docs/
$ make html
```

The generated API docs will be located at python/docs/_build/html/index.html

It can co-exists with those generated by Epydoc.

This is the first working version, after merging in, then we can continue to improve it and replace the epydoc finally.

Author: Davies Liu <davies.liu@gmail.com>

Closes #2292 from davies/sphinx and squashes the following commits:

425a3b1 [Davies Liu] cleanup
1573298 [Davies Liu] move docs to python/docs/
5fe3903 [Davies Liu] Merge branch 'master' into sphinx
9468ab0 [Davies Liu] fix makefile
b408f38 [Davies Liu] address all comments
e2ccb1b [Davies Liu] update name and version
9081ead [Davies Liu] generate PySpark API docs using Sphinx

1 files changed, 3 insertions, 0 deletions

diff --git a/python/pyspark/broadcast.py b/python/pyspark/broadcast.py
index 5c7c9cc161..f124dc6c07 100644
--- a/python/pyspark/broadcast.py
+++ b/python/pyspark/broadcast.py
@@ -78,6 +78,9 @@ class Broadcast(object):
         return self._value
 
     def unpersist(self, blocking=False):
+        """
+        Delete cached copies of this broadcast on the executors.
+        """
         self._jbroadcast.unpersist(blocking)
         os.unlink(self.path)