Document better the "priority" functionality for labels

[alexbruy]

Under Labelling -> Placement, there's a "priority" bar which ranges from "low" to "high". I'm just guessing here, but I suppose this bar is meant to show important the underlying feature is, and therefore whether a label can be placed on it (there's no reference to it under the "help" button help).

Original ticket https://hub.qgis.org/issues/12220

[junek]

Done! I inserted some information into the user manual 2.8.