Explain how ondemand scrub feature works

[mbukatov]

Description

We need to extend description of bitrot detection, so that it contains details about ondemand scrub detection and how to review the result.

What is missing:

• full description of ondemand scrub in section of admin guide, where bit rot detection is discussed
• there is no description of scrub status command anywhere

URLs

• http://docs.gluster.org/en/latest/Administrator%20Guide/Managing%20Volumes/#bitrot-detection
• http://docs.gluster.org/en/latest/release-notes/3.9.0/#on-demand-scrubbing-for-bitrot-detection

See Also

For additional context, see: https://bugzilla.redhat.com/show_bug.cgi?id=1516484#c7