Add comments on MonoDict/TripleDict attributes

src/sage/structure/coerce_dict.pxd

@@ -11,12 +11,13 @@ cdef struct mono_cell:
 @cython.final
 cdef class MonoDict:
     cdef __weakref__
-    cdef size_t mask
-    cdef size_t used
-    cdef size_t fill
+    cdef size_t mask  # size - 1 with 'size' the length of the table array
+    cdef size_t used  # number of valid entries
+    cdef size_t fill  # number of non-NULL entries (including deleted entries)
     cdef mono_cell* table
     cdef bint weak_values
     cdef eraser
+
     cdef mono_cell* lookup(self, PyObject* key)
     cdef get(self, k)
     cdef int set(self, k, value) except -1
@@ -36,12 +37,13 @@ cdef struct triple_cell:
 @cython.final
 cdef class TripleDict:
     cdef __weakref__
-    cdef size_t mask
-    cdef size_t used
-    cdef size_t fill
+    cdef size_t mask  # size - 1 with 'size' the length of the table array
+    cdef size_t used  # number of valid entries
+    cdef size_t fill  # number of non-NULL entries (including deleted entries)
     cdef triple_cell* table
     cdef bint weak_values
     cdef eraser
+
     cdef triple_cell* lookup(self, PyObject* key1, PyObject* key2, PyObject* key3)
     cdef get(self, k1, k2, k3)
     cdef int set(self, k1, k2, k3, value) except -1