Various improvements

Remove `_find` & `_union`.
Remove `DisjointSet_of_integers` `_d` attribute from `DisjointSet_of_hashables` (and adjust code).
Add notes recommending `OP_find` and `OP_union` if no input checking needed.
Touch-up docstrings.

Author: gmou3

src/sage/sets/disjoint_set.pxd

@@ -22,16 +22,13 @@ cdef class DisjointSet_class(SageObject):
 cdef class DisjointSet_of_integers(DisjointSet_class):
     cpdef int find(self, int i)
     cpdef void union(self, int i, int j)
-    cdef inline int _find(self, int i)
-    cdef inline void _union(self, int i, int j)
     cpdef root_to_elements_dict(self)
     cpdef element_to_root_dict(self)
     cpdef to_digraph(self)
 
 cdef class DisjointSet_of_hashables(DisjointSet_class):
     cdef list _int_to_el
     cdef dict _el_to_int
-    cdef DisjointSet_of_integers _d
     cpdef find(self, e)
     cpdef void union(self, e, f)
     cpdef root_to_elements_dict(self)