Update Lua API documentation to include minetest.get_modnames()

doc/lua_api.txt

@@ -718,6 +718,8 @@ minetest namespace reference
 minetest.get_current_modname() -> string
 minetest.get_modpath(modname) -> eg. "/home/user/.minetest/usermods/modname"
 ^ Useful for loading additional .lua modules or static data from mod
+minetest.get_modnames() -> list of installed mods
+^ Return a list of installed mods, sorted alphabetically
 minetest.get_worldpath() -> eg. "/home/user/.minetest/world"
 ^ Useful for storing custom data
 minetest.is_singleplayer()