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