Examples should cover XML writing more thoroughly

[dralley]

Currently we have many examples of XML reading, and nothing for XML writing.

[Mingun]

I started to write some docstring examples (well, almost couple of years ago), but realized, that we should fix our constructors. Currently we trust to users that the strings passed to constructors (for PI, CData, or Comment in particular) does not contain end sequences. We should instead:

• wrap data only up to ?> for PI and return error. If user is fine with that it can use returned event;
• wrap data only up to --> for Comment and return error. If user is fine with that it can use returned event;
• construct an iterator over CData content if we find ]]> in the data:

  impl BytesCData {
    // abcd]]>efgh => "abcd]]" + ">efgh"
    pub fn new(content: &str) -> impl Iterator<Item = Self> { ... }
  }

That is one reason why I want to split reader events from writer events