[SPARK-6266] [MLLIB] PySpark SparseVector missing doc for size, indices, values

Write missing pydocs in `SparseVector` attributes.

Author: lewuathe <lewuathe@me.com>

Closes #7290 from Lewuathe/SPARK-6266 and squashes the following commits:

51d9895 [lewuathe] Update docs
0480d35 [lewuathe] Merge branch 'master' into SPARK-6266
ba42cf3 [lewuathe] [SPARK-6266] PySpark SparseVector missing doc for size, indices, values

python/pyspark/mllib/linalg.py

@@ -445,8 +445,10 @@ class SparseVector(Vector):
         values (sorted by index).
 
         :param size: Size of the vector.
-        :param args: Non-zero entries, as a dictionary, list of tupes,
-               or two sorted lists containing indices and values.
+        :param args: Active entries, as a dictionary {index: value, ...},
+          a list of tuples [(index, value), ...], or a list of strictly i
+          ncreasing indices and a list of corresponding values [index, ...],
+          [value, ...]. Inactive entries are treated as zeros.
 
         >>> SparseVector(4, {1: 1.0, 3: 5.5})
         SparseVector(4, {1: 1.0, 3: 5.5})
@@ -456,6 +458,7 @@ class SparseVector(Vector):
         SparseVector(4, {1: 1.0, 3: 5.5})
         """
         self.size = int(size)
+        """ Size of the vector. """
         assert 1 <= len(args) <= 2, "must pass either 2 or 3 arguments"
         if len(args) == 1:
             pairs = args[0]
@@ -463,7 +466,9 @@ class SparseVector(Vector):
                 pairs = pairs.items()
             pairs = sorted(pairs)
             self.indices = np.array([p[0] for p in pairs], dtype=np.int32)
+            """ A list of indices corresponding to active entries. """
             self.values = np.array([p[1] for p in pairs], dtype=np.float64)
+            """ A list of values corresponding to active entries. """
         else:
             if isinstance(args[0], bytes):
                 assert isinstance(args[1], bytes), "values should be string too"