Mention attachment title redactions in documentation

[WilliamWDTK]

It has been noted (on WDTK Slack) that the fact (according to @RichardTaylor) that redactions also apply to attachment filenames is:

• non-obvious and perhaps unexpected, and
• not documented on https://alaveteli.org/docs/running/redaction/

[garethrees]

I'd actually invert this a little in that I think our base assumption should be that censor rules apply to all user-generated content on a request page. We do have one case where this isn't yet true (https://github.com/mysociety/alaveteli/issues/1881), but that's the only outlier I think.

Given that framing, do you have a suggested phrasing that would make this clearer on the redaction page?