Doc. Complete best with disks section

doc/source/architecture.rst

@@ -588,7 +588,7 @@ Each space can have **weight** variable specified (**1** by default), which is u
     x_3 + x_4 \le 200 \\
     x_1 + x_3 = 100 \\
     x_2 + x_4 = 100 \\
-    x_2 * (1 / weight) + x_4 * (-1 / weight) = 0
+    x_2 * (1 / weight) - x_4 * (1 / weight) = 0
     \end{cases}
 
 To satisfy last equality, spaces have to be equal in size.
@@ -646,8 +646,29 @@ In order to do that lets make order coefficient :math:`0 < \textrm{order coeffic
 .. math::
 
    c = \begin{bmatrix}
-   1 + (1/9) &
-   0 + (1/6) &
-   0 + (1/4) &
-   1 + (1/2)
-   \end{bmatrix}^{T}\\[2ex]
+   1 + (1/2) \\
+   0 + (1/4) \\
+   0 + (1/6) \\
+   1 + (1/9)
+   \end{bmatrix}
+
+#. build sets according to selected disks, in our case we have two sets, **hdd** and **ssd** disks
+#. for spaces which belong to specific set of disks add **1** to a coefficient which represents this space on a disk from the set
+#. if space does not belong to the set of disks, add **0**
+
+To make sure that spaces are always (unless size constraints are not violated), going to be allocated on the disks which they best suited with,
+we automatically add a special artificial volume **unallocated**, whose coefficient is always **1**, and in this case we should change
+coefficient of space which belongs to set of disks to **2**.
+
+.. math::
+
+   c = \begin{bmatrix}
+   2 + (1/2)\\
+   0 + (1/4)\\
+   1 \\
+   0 + (1/9)\\
+   2 + (1/12)\\
+   1
+   \end{bmatrix}
+
+As the result if space has one or more **best_with_disks**, it will be allocated on the disks only.