Add the expected behaviours of Mountpoint in the troubleshooting page that customers face

[sauraank]

Tell us more about this new feature.

There are cases which are expected behaviour for Mountpoint but customers might find it surprising.

• Directory (S3 prefix) disappear when we delete all the files contained within than prefix.
  This is an expected behaviour as S3 do not support directories and objects are just grouped using prefix appearing like Directory. We have explained it in Semantics document

We would like these cases to be reported in troubleshooting page as well so the customer know about these cases easily and be able to distinguish it from actual errors.
This is to address earlier issue faced by customer: #682

[dannycjones]

We added new troubleshooting documentation in #703 and added details on the shadowing issue in #789.