The Collaborative International Dictionary of English v.0.48:

Explanatory \Ex*plan"a*to*ry\, a. [L. explanatorius.]
   Serving to explain; containing explanation; as explanatory
   notes. --Swift. Explat

WordNet (r) 3.0 (2006):

explanatory
    adj 1: serving or intended to explain or make clear;
           "explanatory notes"; "an explanatory paragraph"

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "explanatory":
   allegorizing, annotative, clarifying, critical, demonstrative,
   demythologizing, descriptive, discursive, editorial, elucidative,
   enlightening, euhemeristic, exegetic, exegetical, exemplificative,
   explaining, explicative, explicatory, expositional, expositive,
   expository, glossarial, illuminating, illuminative, illustrative,
   interpretative, interpretive, rationalistic, rationalizing,
   revelatory, scholiastic