diff --git a/doc/client_lua_api.md b/doc/client_lua_api.md
index 3bd1e3567..041c6eaa8 100644
--- a/doc/client_lua_api.md
+++ b/doc/client_lua_api.md
@@ -408,6 +408,7 @@ examples.
 * Clickable button. When clicked, fields will be sent.
 * `x`, `y` and `name` work as per field
 * `w` and `h` are the size of the button
+* Fixed button height. It will be vertically centred on `h`
 * `label` is the text on the button
 * Position and size units are inventory slots
 
diff --git a/doc/lua_api.txt b/doc/lua_api.txt
index e675cc703..3eb1fdc64 100644
--- a/doc/lua_api.txt
+++ b/doc/lua_api.txt
@@ -1857,6 +1857,7 @@ examples.
 * Clickable button. When clicked, fields will be sent.
 * `x`, `y` and `name` work as per field
 * `w` and `h` are the size of the button
+* Fixed button height. It will be vertically centred on `h`
 * `label` is the text on the button
 * Position and size units are inventory slots