added config option favor_generated for @return

added a config option favor_generated - if true (the default), the
    generated comment's @return tag (and possibly other tags in the
    future) overrides what is in the source file.  Otherwise, the source
    file @return or @returns tag is kept.  The reason one might want to
    keep the source file's @return is that it might have additional
    description about what is being returned beyond just the type.