doc: fix WARNING: Invalid definition" due to unamed structs/unions

Fix with a workaround in unnamed unions / structs in bluetooth, i2c,
sensor and uart.

Current documentation parsers (sphinx under Doxygen) don't seem to
understand well unnamed structs / unions. They will not generate any
documentation for any documented members (see left side of
http://imgur.com/mcpBXWc).

A workaround is to make the parser think there is something like a
struct/union/enum name that is actually something with no effect to
the compiler.

Naming it with __unnamed_workaround__ ensures it is clear it is a
workaround while we wait for a final fix. It is #defined to be a NO-OP
to the compiler and rearrange the member documentation as *@param* so
we have some documentation that the non-worked around code fails to
document.

Anonymous structs/union that declare a variable are just given an
internal name.

Workarounds documented in the contribution guidelines.

Change-Id: I4d32cf444f3c5e7d2fb11581e4b41f80e93c9786
Signed-off-by: Inaky Perez-Gonzalez <inaky.perez-gonzalez@intel.com>

include/bluetooth/conn.h

@@ -135,19 +135,22 @@ enum {
 	BT_CONN_ROLE_SLAVE,
 };
 
-/** Connection Info Structure */
+/** @brief Connection Info Structure
+ *
+ *
+ *  @param type Connection Type
+ *  @param role Connection Role
+ *  @param le LE Connection specific Info
+ *  @param br BR/EDR Connection specific Info
+ */
 struct bt_conn_info {
-	/** Connection Type */
 	uint8_t type;
 
-	/** Connection Role */
 	uint8_t role;
 
-	union {
-		/** LE Connection specific Info */
+	union __unnamed_workaround__ {
 		struct bt_conn_le_info le;
 
-		/** BR/EDR Connection specific Info */
 		struct bt_conn_br_info br;
 	};
 };