Add more information about storage options in Zaqar config-ref

[Eva Balycheva]

The Message service Configuration Reference needs to provide more information about options in storage drivers options in zaqar.conf.

For example, this information can be added:
1. The information about which storage options are active depending on the chosen storage back end.
2. The general information about storage pipelines.
3. Currently possible values (stages) for each one of the storage pipelines.
4. The information on how to enable notification feature of Zaqar by adding "zaqar.notification.notifier" stage to the "message_pipeline" option.
5. A brief information about possibility to add external stages to the storage pipelines.

Typos in some articles should be fixed:
1. A word "backend" should be replaced to "back end".

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/268860

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/268860
Committed: https://git.openstack.org/cgit/openstack/openstack-manuals/commit/?id=d1ce72743597752f3dd2bf27774704927d437080
Submitter: Jenkins
Branch: master

commit d1ce72743597752f3dd2bf27774704927d437080
Author: Eva Balycheva <email address hidden>
Date: Sat Jan 16 05:53:52 2016 +0300

    More info in Zaqar config-ref storage options

    This patch adds some important information about zaqar's (Message
    service) storage options:

    1. The information about which storage options are active depending on
    the chosen storage back end.
    2. The general information about storage pipelines.
    3. Currently possible values (stages) for each one of the storage
    pipelines.
    4. The information on how to enable notifications feature of zaqar.
    5. A brief information about possibility to add external stages to the
    storage pipelines.

    Some typos are fixed by patch:
    In some articles the word "backend" is replaced to "back end".

    Change-Id: Iffc9ea9dcefdb76e15554e9223e9e8dbbe8288cd
    Closes-Bug: 1534852

[OpenStack Infra (hudson-openstack)]

This issue was fixed in the openstack/openstack-manuals 15.0.0 release.