edited awkward phrasing. changed 'local copies' to 'cache locally'.

At least on my computer (running Fedora Linux and Darktable 1.4.2), the
button in Selected Images panel in lighttable mode is labeled 'cache
locally', not 'local copies', so I changed the reference in the manual
page here to be more consistent.
Merge branch 'master' of https://github.com/darktable-org/darktable into clarifying-text-in-user-manual
further tweaking of prose in localcopies.xml

Following the discussion
[here](http://www.darktable.org/redmine/issues/9955)
I changed my "local cache" edits back to "local copies." The feature
name change should be reflected in the application source too.

I changed some references to "external storage" to merely "main
storage" and the like, because I think the use case for this feature
applies to any sort of storage that would be impractical to carry
around, not just to RAID NAS.
edited prose for readability. added some extra information about feature.

A couple times I changed "multiple images" to "one or more images,"
because the rating and labeling methods for multiple images work just
as well for single images.
fixing a few things I missed.
minor typography update: changed ` - ` to actual emdash (`&mdash;`).
edited for clarity and readability.
Merge pull request #565 from Terrycloth/clarifying-text-in-user-manual

Third batch of edits to user manual. (See individual commits for details.)