Document maxbuildheight variable

Signed-off-by: Patrick <cowinkkeydinkinc@gmail.com>

docs/modules/mechanics/variables.mdx

@@ -20,6 +20,7 @@ You can define as many variables as you want and all variables must have a scope
   | `<variable/>` | An individual variable. |
   | `<score/>` | A score variable, allowing direct access to competitor's score. This is automatically scoped to teams. |
   | `<timelimit/>` | A time limit variable which starts/stops the time limit of the match. |
+  | `<maxbuildheight/>` | A variable that sets the build height of the map. |
   | `<with-team/>` | A team-rescoping variable that allows using a specific team's value from a different team-scoped variable as a match-scoped variable. |
 </div>
 
@@ -156,6 +157,22 @@ can be ended by setting the value to `0`.
 </actions>
 ```
 
+### Maxbuildheight Variable
+The `maxbuildheight` variable will give a height limit for placing blocks
+after being set to a certain value.
+
+```xml
+<variables>
+    <maxbuildheight id="build-limit"/>
+</variable>
+<actions>
+    <action id="increase-buildheight" scope="match" expose="true">
+        <!-- Adds 1 to the current height, up to a maximum of 55 -->
+        <set var="build-limit" value="min(55,whatever+1"/>
+    </action>
+</actions>
+```
+
 ### Team-Rescoping Variables
 Whenever `some-filter` matches for a player (e.g. entering a region), it will trigger an action that sets red team's score to exactly 10, then adds 5 to the score of the team the player is in.
 In short, if a player in red team triggers `some-filter`, red is reset to 10 + 5 = 15, no other scores are affected.