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?
Yes, thinking about you are right - though finding the cause of the failure may not be straightforward. This was cut&paste from the `ln_rm` section. Should it be regarded as a similar advantage there?

/cc @joshmoore