drm: Update connector documentation

- Shuffle docs from drm-kms.rst into the structure docs where it makes
  sense.
- Put the remaining bits into a new overview section.

One thing I've changed is around probing: Old docs says that you
_must_ use the probe helpers, which isn't correct. Helpers are always
optional.

v2: Review from Sean.

Cc: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1471034937-651-17-git-send-email-daniel.vetter@ffwll.ch

drivers/gpu/drm/drm_connector.c

@@ -27,6 +27,37 @@
 #include "drm_crtc_internal.h"
 #include "drm_internal.h"
 
+/**
+ * DOC: overview
+ *
+ * In DRM connectors are the general abstraction for display sinks, and include
+ * als fixed panels or anything else that can display pixels in some form. As
+ * opposed to all other KMS objects representing hardware (like CRTC, encoder or
+ * plane abstractions) connectors can be hotplugged and unplugged at runtime.
+ * Hence they are reference-counted using drm_connector_reference() and
+ * drm_connector_unreference().
+ *
+ * KMS driver must create, initialize, register and attach at a struct
+ * &drm_connector for each such sink. The instance is created as other KMS
+ * objects and initialized by setting the following fields.
+ *
+ * The connector is then registered with a call to drm_connector_init() with a
+ * pointer to the connector functions and a connector type, and exposed through
+ * sysfs with a call to drm_connector_register().
+ *
+ * Connectors must be attached to an encoder to be used. For devices that map
+ * connectors to encoders 1:1, the connector should be attached at
+ * initialization time with a call to drm_mode_connector_attach_encoder(). The
+ * driver must also set the struct &drm_connector encoder field to point to the
+ * attached encoder.
+ *
+ * For connectors which are not fixed (like built-in panels) the driver needs to
+ * support hotplug notifications. The simplest way to do that is by using the
+ * probe helpers, see drm_kms_helper_poll_init() for connectors which don't have
+ * hardware support for hotplug interrupts. Connectors with hardware hotplug
+ * support can instead use e.g. drm_helper_hpd_irq_event().
+ */
+
 struct drm_conn_prop_enum_list {
 	int type;
 	const char *name;
@@ -77,7 +108,7 @@ void drm_connector_ida_destroy(void)
  * drm_connector_get_cmdline_mode - reads the user's cmdline mode
  * @connector: connector to quwery
  *
- * The kernel supports per-connector configration of its consoles through
+ * The kernel supports per-connector configuration of its consoles through
  * use of the video= parameter. This function parses that option and
  * extracts the user's specified mode (or enable/disable status) for a
  * particular connector. This is typically only used during the early fbdev