Document features and limitations.

[aercolino]

As it appears, the Regex class (in particular, but any other too) is still pretty immature and it only allows for a very limited set of features. It's a good time to start documenting organically what is allowed and what is not, also taking into account the many TODOs / issues.

[aercolino]

One big feature and limitation at the same time is that the Ando_Regex class only allows to compose well formed expressions. For example, (aa ($b) cc) with b => <bb> is ok, but with b => ?<bb> is not. In the former case <bb> is a meaningful regular expression (well formed), while in the latter ?<bb> is not only a broken regular expression by itself (not well formed), but it also matches something completely different when it's used alone and when it's put in place into the other. (see also #11)