Document "default mode" queries

[goodmami]

In #157 I noticed that the "default mode" for queries is not documented, even though it is mentioned in the CHANGELOG and in some issues like #90 and #92. It should be explained at least once in the documentation and referred to from some relevant places (Wordnet class API docs, Basic Usage and Interlingual Queries guides, maybe others). I have, however, been trying to emphasize the module functions less in favor of creating Wordnet objects, so the description would be better with some admonishment to use Wordnet objects in most cases (e.g., with a warning that path methods in default-mode can be very slow when many lexicons are installed).

Note that default mode is enabled when either (a) a module function like wn.synsets() is used without any lexicon or lang parameter, or (b) a Wordnet object is created without any lexicon or lang parameter.