Example modified and docs added

// edited by skirpichev

Based on sympy/sympy#12240

Signed-off-by: Sergey B Kirpichev <skirpichev@gmail.com>

diofant/solvers/bivariate.py

@@ -36,15 +36,18 @@ def _mostfunc(lhs, func, X=None):
     ``func`` can be a function (exp, log, etc...) or any other Diofant object,
     like Pow.
 
+    If ``X`` is not ``None``, then the function returns the term composed with the
+    most ``func`` having the specified variable.
+
     Examples
     ========
 
     >>> _mostfunc(exp(x) + exp(exp(x) + 2), Pow)
     E**(E**x + 2)
     >>> _mostfunc(exp(x) + exp(exp(y) + 2), Pow, x)
     E**x
-    >>> _mostfunc(exp(x) + exp(exp(y) + 2), Pow, x)
-    E**x
+    >>> _mostfunc(exp(x) + exp(exp(y) + 2), Pow)
+    E**(E**y + 2)
     >>> _mostfunc(x, exp, x) is None
     True
     >>> _mostfunc(exp(x) + exp(x*y), Pow, x)