tree.pyx: Add doc for Tree.to_str().

So it appears in the class Tree sphinx documentation.
Even if it is redundant with the doc in text_vix.py.

ete4/core/tree.pyx

@@ -304,6 +304,15 @@ cdef class Tree(object):
 
     def to_str(self, show_internal=True, compact=False, props=None,
                px=None, py=None, px0=0, waterfall=False):
+        """Return a string containing an ascii drawing of the tree.
+
+        :param show_internal: If True, show the internal nodes too.
+        :param compact: If True, use exactly one line per tip.
+        :param props: List of node properties to show. If None, show all.
+        :param px, py, px0: Paddings (x, y, x for leaves). Overrides `compact`.
+        :param waterfall: Use a waterfall representation. Overrides
+            `show_internal`, `compact`, `px`, `py`, `px0`.
+        """
         return text_viz.to_str(self, show_internal, compact, props,
                                px, py, px0, waterfall)