Add guideline about annotation using one-line

From https://github.com/owncloud/core/pull/12279#discussion_r20546875

Opinions:
- @blizzz: 1 use-statement/relative
- @butonic: 1 use-statement - switch to @type as PSR5
- @Raydiation: 1 use-statement - switch to @type as PSR5
- @icewind1991: 3 full qualified
- @LukasReschke: ~~1 full qualified~~ 1 use-statement - switch to @type as PSR5
- @MorrisJobke: 1 full qualified
- @nickvergessen: 1 line
Probably bad idea to use a private namespace class