More formal definition of functions in documentation

Dear,

I'm currently reading through the Postgresql documentation about how
several functions work and which ones I would need. So far the
documentation is great and well-structured!

Unfortunally I'm not sure what functions will actually do when some
non-obvious input is provided (which is sometimes required, as we cannot
assume structure of the input at all times). A simple example is
substring(text from 'blaat#"%#"' for '#')
where text is not in the format of the regular expression, e.g. when
text = 'text'. I don't know if the SQL standard includes such
requirements, but documenting such behaviour might be a good addition to
the help.
If I know how and how to structure it, I'm willing to do some effect.

In any case, something as already done for CREATE TABLE and such
constructs are very good and well-defined.

- Joris