Update pipeline.rst

Three new pipeline functions were added into the default pipeline between versions 0.1.11 and 0.1.12. If customizing pipeline, those 3 are needed

https://github.com/omab/python-social-auth/commit/ef7b22d066597eb36765c0f7daddfc9d89f74e52#L4R134
Merge pull request #31 from jontsai/patch-1

Update pipeline.rst
Update pipeline docs with disconnection-pipeline feature