Fourth batch of edits to user manual.

After the discussion [here](https://github.com/darktable-org/darktable/pull/565#issuecomment-43915550), *em* dashes were [changed](https://github.com/darktable-org/darktable/commit/d318f957397ab5a1deec5746e5e741e93c1cf836) to *en* dashes. I replaced the space to the left of the en dashes with a non-breaking space.

In the section about the lighttable *collect images* panel, I tried to rephrase any instances of developer jargon (e.g., "substrings") for lay users, plus clean up the prose at a more general level.