Expose upload_rm option in the documentation

This is a doc change explaining the `upload_rm` cli import option. It was originally written as a 5.0.4 fix and so made minimal changes to pages. It may be that for 5.1 we will want to further re-organise the import pages.

@sbesson This needs the file `/downloads/inplace/advanced-help.txt` pulling in from somewhere. Is there an automatic way to do this?
This doesn't exactly feel like a disadvantage to be honest. The other advantage is that it leaves the failed imports behind so you can see what they were and that you don't lose the data!