rpmsg: Add short description of the IOCTL defined in UAPI.

Add a description of the IOCTLs and provide information on the default
value of the source and destination addresses.

Reviewed-by: Bjorn Andersson <[e-mail]>
Reviewed-by: Mathieu Poirier <[e-mail]>
Signed-off-by: Arnaud Pouliquen <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Bjorn Andersson <[e-mail]>

include/uapi/linux/rpmsg.h

···
 /**
  * struct rpmsg_endpoint_info - endpoint info representation
  * @name: name of service
- * @src: local address
- * @dst: destination address
+ * @src: local address. To set to RPMSG_ADDR_ANY if not used.
+ * @dst: destination address. To set to RPMSG_ADDR_ANY if not used.
  */
 struct rpmsg_endpoint_info {
 	char name[32];
···
 	__u32 dst;
 };
 
+/**
+ * Instantiate a new rmpsg char device endpoint.
+ */
 #define RPMSG_CREATE_EPT_IOCTL	_IOW(0xb5, 0x1, struct rpmsg_endpoint_info)
+
+/**
+ * Destroy a rpmsg char device endpoint created by the RPMSG_CREATE_EPT_IOCTL.
+ */
 #define RPMSG_DESTROY_EPT_IOCTL	_IO(0xb5, 0x2)
 
 #endif