Extend the `filter()` documentation slightly

[The-Mule]

Even though the original filter description is correct and does not contain any factual flaws it might be confusing to people. There are two improvements that I propose:

1. People tend to try using parentheses in their filter expressions for obvious reasons. But it is not possible. I feel like this should be mentioned in the documentation. In fact, with DNF you never need parentheses to express any expression and I think this fact also needs to be noted there.

2. Filter supports regular expressions but currently there is a limitation by design (a limitation that can actually be removed). Use of '|' and '&' in the regexp is not allowed - it leads to filter error. This should be fixed but until then it'd better be documented.