mail-lua: Document %{lua_file} and %{lua_call}

docs/core/plugins/mail_lua.md

@@ -1,6 +1,8 @@
 ---
 layout: doc
 title: mail-lua
+dovecotlinks:
+  var_expand_lua: Variable expansion functions
 ---
 
 # Mail Lua Plugin (`mail-lua`)
@@ -13,3 +15,58 @@ See [[link,lua]].
 ## Settings
 
 <SettingsComponent plugin="mail-lua" />
+
+## Variable expansion functions
+
+This component registers two providers. Note that these providers have very
+limited functionality at this moment.
+
+### `%{lua_file(<path>,<function>,...)}`
+
+Executes the script at given path, and if it's successful, use the string
+result. If values are provided after function name, they are passed to
+the Lua function as parameters. Pipeline value is always added last, if present.
+
+Example:
+```[dovecot.conf]
+mail_plugins {
+  mail_lua = yes
+}
+
+mail_home = %{user|lua_file("/etc/dovecot/get_home.lua","get_home")}
+```
+
+```[get_home.lua]
+-- This is not a good way to do this, provided here for example
+-- purposes only. Do not use this.
+
+local function get_home(user)
+  return "/home/" .. user
+end
+```
+
+### `%{lua_call(<function>,...)}`
+
+Executes Lua function in the [[setting,lua_file]], and if successful, use the string result. If values are provided after function name, they are passed to
+the Lua function as parameters. Pipeline value is always added last, if present.
+
+Example:
+```[dovecot.conf]
+mail_plugins {
+  mail_lua = yes
+}
+
+mail_lua {
+ lua_file = /etc/dovecot/mail.lua
+}
+mail_home = %{user|lua_call("get_home")}
+```
+
+```[mail.lua]
+-- This is not a good way to do this, provided here for example
+-- purposes only. Do not use this.
+
+local function get_home(user)
+  return "/home/" .. user
+end
+```