Doc: edit devicesupport plugin API docs

Edit for grammar and style. Remove \brief commands from
function descriptions.

Move function descriptions to directly above the functions in
code, so that the \fn command can be removed and need not be
maintained.

Change-Id: Idb5bdd1eccf657be689f9d6d1d7eb1ca36f8f1b7
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>