wlr/backend/x11.h: add documentation

Copied from wlr/backend/wayland.h and modified accordingly.

include/wlr/backend/x11.h

@@ -9,12 +9,37 @@
 #include <wlr/types/wlr_input_device.h>
 #include <wlr/types/wlr_output.h>
 
+/**
+ * Creates a new wlr_x11_backend. This backend will be created with no outputs;
+ * you must use wlr_x11_output_create to add them.
+ *
+ * The `x11_display` argument is the name of the X Display socket. Set
+ * to NULL for the default behaviour of XOpenDisplay.
+ */
 struct wlr_backend *wlr_x11_backend_create(struct wl_display *display,
 	const char *x11_display, wlr_renderer_create_func_t create_renderer_func);
+
+/**
+ * Adds a new output to this backend. You may remove outputs by destroying them.
+ * Note that if called before initializing the backend, this will return NULL
+ * and your outputs will be created during initialization (and given to you via
+ * the output_add signal).
+ */
 struct wlr_output *wlr_x11_output_create(struct wlr_backend *backend);
 
+/**
+ * True if the given backend is a wlr_x11_backend.
+ */
 bool wlr_backend_is_x11(struct wlr_backend *backend);
+
+/**
+ * True if the given input device is a wlr_x11_input_device.
+ */
 bool wlr_input_device_is_x11(struct wlr_input_device *device);
+
+/**
+ * True if the given output is a wlr_x11_output.
+ */
 bool wlr_output_is_x11(struct wlr_output *output);
 
 #endif