DOC: update .str.contains/match/startswith docstring examples for default  behaviour
Updating the docstrings of `.str.` predicate methods that have the `na` keyword. 

For the examples, the current text is no longer correct (because the default behaviour with str dtype is now to already return False). 
For now I just removed those examples. I could instead update the example to create an object-dtype Series to still show the `na` behaviour, but personally I feel that would make the docstring examples more complex than needed, and that it is fine to let they focus on just the default dtype. But no strong opinion ;)
Thanks @jorisvandenbossche 