Rewrite section about corrupt index

- Add clear note about upgraded 4.4.x servers
- Do not mention loosely defined "corrupt" index
- Add reference to troubleshooting

OME-DCO-1.1-Signed-off-by: Sebastien Besson <seb.besson@gmail.com> (github: sbesson)
Rename re-indexing monitoring section

OME-DCO-1.1-Signed-off-by: Sebastien Besson <seb.besson@gmail.com> (github: sbesson)