DockerfileLintBear.py: Improve docstring
DockerfileLintBear.py: Improve docstring

Closes https://github.com/coala/coala-bears/issues/2259

coala-analyzer/coala-bears,coala-analyzer/coala-bears,refeed/coala-bears,coala-analyzer/coala-bears,coala/coala-bears,coala/coala-bears,refeed/coala-bears,coala/coala-bears,coala/coala-bears,coala/coala-bears,coala/coala-bears,refeed/coala-bears,refeed/coala-bears,refeed/coala-bears,refeed/coala-bears,coala-analyzer/coala-bears,refeed/coala-bears,coala-analyzer/coala-bears,coala/coala-bears,coala/coala-bears,refeed/coala-bears,coala-analyzer/coala-bears,coala-analyzer/coala-bears,coala/coala-bears,refeed/coala-bears,refeed/coala-bears,coala/coala-bears,coala/coala-bears,coala-analyzer/coala-bears,coala/coala-bears,coala-analyzer/coala-bears,refeed/coala-bears