It's an honorific, meaning "illustrious or enlightened general", and as such is not specific to Liu Bei. 

For example, in the San guo zhi, Pei Song's annotations cite a passage where Huangfu Li (皇甫酈) addresses Li Jue (李傕) as 明將軍: 

近董公之強，明將軍目所見. 
In recent times, you yourself, illustrious General, saw with your own eyes how powerful Dong Zhuo was ...