Add "MAX" & "MIN" modType explanation to docs

"MAX" already existed and was used, but was missing from `modSyntax.md`,
so I added the explanation for both now

Author: majochem

docs/modSyntax.md

@@ -13,6 +13,7 @@ Used as a key, so you can reference this mod elsewhere in PoB.  Can really be an
 - "OVERRIDE": used when you want to ignore any calculations done on this mod and just use the value (e.g. "your resistances are 78%" from Loreweave)
 - "FLAG": used for conditions.  Value will be true/false when this type is used.
   - When you need the "FLAG" ModType, consider using the function `flag(name, source, modFlags, keywordFlags, extraTags)` instead. This method shortens the code and clarifies the intent. For example, `flag("ZealotsOath", { type = "Condition", var = "UsingFlask" })` is the same as `mod("ZealotsOath", "FLAG", true, { type = "Condition", var = "UsingFlask" })`
+- "MAX" and "MIN": used for values where only the highest or lowest value should take effect respectively. Examples are `"ImprovedMinionDamageAppliesToPlayer"` for "Increases and Reductions to Minion Damage apply ... at X% of their value" or `"PoisonStackLimit"` for "Cannot Poison Enemies with at least X Poisons on them"
 ### Value
 This represents the raw value of the mod.  When it's used in the skills to map from the skill data, this will be `nil`, as it pulls the number from the gem based on the level.
 ### Source