Update Arena Setup documentation

This commit updates the Arena Setup documentation.

* Follows RST document style guide
* Uses active voice / minor edits
* Improve document organization

I also renamed the wave-formulas.rst file since it had a typo, and was
referenced in the Arena Setup document.

docs/user/wave-formulas.rst

@@ -0,0 +1,92 @@
+#############
+Wave formulas
+#############
+
+This page holds an overview of all the formulas used in the MobArena
+waves system. Customizing the different properties of the waves should
+be somewhat easier if they can be calculated, so here they all are!
+
+About notation: Each variable used in the formulas will have its own
+name. A variable that starts with a ``#`` denotes "number (of)", so
+``#players`` means "number of players", and ``#wave`` means "wave
+number". The function ``min(a,b)`` returns the lowest of the values
+``a`` and ``b``, and ``max(a,b)`` returns the highest.
+
+Wave growth
+~~~~~~~~~~~
+
+The wave growth node ``growth``, used in default waves, denotes how fast
+monster amounts grow over time. The base is calculated by half of the
+number of players, but at most 13 (i.e. there is no difference between
+25 and 50 players). The amounts can be altered further using the
+``amount-multiplier`` (see the [[wave setup page\|Setting up the
+waves]]).
+
+::
+
+    #monsters = base * #wave^exp
+    base = min(#players/2 + 1 , 13)
+
+The ``exp`` variable is defined by the growth node, and has the
+following values:
+
+::
+
+    slow   = 0.5
+    medium = 0.65
+    fast   = 0.8
+    psycho = 1.2
+
+Note that with the node value ``old`` (which is the default), the
+monster count is ``#wave + #players``.
+
+Swarm Amount
+~~~~~~~~~~~~
+
+The swarm amount node ``amount``, used in swarm waves, denotes how many
+monsters should spawn in the swarm waves. There will always be at least
+10 monsters due to the max function and the lowest multiplier value
+being 10, however this can be further customized with the
+``amount-multiplier`` (see the [[wave setup page\|Setting up the
+waves]]).
+
+::
+
+    #monsters = max(1, #players/2) * multiplier
+
+The ``multiplier`` variable is defined by the amount node, and has the
+following values:
+
+::
+
+    low    = 10
+    medium = 20
+    high   = 30
+    psycho = 60
+
+Boss Health
+~~~~~~~~~~~
+
+The boss health node ``health``, used in boss waves, denotes how much
+health the boss has. Note that the ``health-multiplier`` node (see the
+[[wave setup page\|Setting up the waves]]) **does NOT** affect boss
+waves at all, so these are the only values that can be used. The minimum
+health a boss can have is 320 health points (~160 hearts), which is with
+``low`` health and only 1 player fighting. With 10 players and ``high``
+health, the boss will have 5500 health points (~2750 hearts).
+
+::
+
+    health = (#players + 1) * 20 * multiplier
+
+The ``multiplier`` variable is defined by the health node, and has the
+following values:
+
+::
+
+    verylow   = 4
+    low       = 8
+    medium    = 15
+    high      = 25
+    veryhigh  = 40
+    psycho    = 60