Second (small) edit to user manual

There is some inconsistency in the docs, where sometimes the feature in the *selected images* panel of lighttable mode is referred to as "local copies" and sometimes it's called "local cache" or "cache locally." 

Here, my edits in `doc/usermanual/lighttable/concepts/localcopies.xml` chanced "local copies" to "cache locally," to match the actual label on the button in Darktable. However, the section title, section id, and file name still say "local copies." It seems to me that it would be better to change the title, id, file name, and all references to these to be "local *cache*" or something like that, to keep everything consistent. But I wanted to get some feedback before I changed things that much.

I also notice the use of `local_copies` in various `src` files. For the sake of consistency, it might help to also rename those functions, but I don't know C/++ well enough to feel comfortable making such changes.

What do you think?