Documentation: Fix various inconsistencies in documentation

Pivotal Tracker: 58478260

Additional authors:
    - Hai Qian <hqian@gopivotal.com>
    - Shengwen Yang <syang@gopivotal.com>
    - Xixuan Feng <xfeng@gopivotal.com>

Changes:
    - Complete the Release notes
    - Gppkg version number set to 1.8
    - Fix various documentation errors in multiple modules
    - Changed incorrect function declaration in margins_mlogregr