Reword storage docs on corruption issues

[SuperQ]

Reword the section on what to do if major corruption happens. Users are being confused by the existing wording and cherry-picking the meaning from the single sentance about durability.

[roidelapluie]

Should chunks_head be removed of the wall is removed?

[bboreham]

We may have wal, wbl and chunks_head directories.

[SuperQ]

@roidelapluie I don't know

@bboreham Do you have any suggested wording changes?

[roidelapluie]

cc @jesusvazquez