Documentation: Update the doxygen tagline

* Make it the same as the one used on the website and elsewhere
darwin: squash warning about returning size_t as int

I also updated the documentation on the backend get_config_descriptor
function. The documentation incorrectly stated that this function returns
0 on success when it really returns the length of the descriptor.

Signed-off-by: Nathan Hjelm <hjelmn@me.com>
Enabled additional warnings in Xcode project
Removed empty \see statements, fixing -Wdocumentation warnings
Enabled LTO optimization in release in Xcode project
Removed obsolete comment related to libusbx fork