host: document GATT error status encoding

Clarify that ble_gatt_error.status uses NimBLE host return-code
ranges instead of raw ATT error values.

This helps callers decode application-specific ATT errors such as
0x0181 by first removing the ATT base.

Author: aineoae86-sys

nimble/host/include/host/ble_gatt.h

@@ -204,7 +204,15 @@ struct ble_hs_cfg;
 /*** @client. */
 /** Represents a GATT error. */
 struct ble_gatt_error {
-    /** The GATT status code indicating the type of error. */
+    /**
+     * The NimBLE host status code indicating the type of error.
+     *
+     * This value uses the NimBLE host return-code ranges. ATT errors are
+     * reported as BLE_HS_ATT_ERR(code), so status 0x0181 is ATT error 0x81
+     * (0x100 + 0x81). Application-specific ATT errors must be decoded by
+     * subtracting the ATT base and consulting the service or profile
+     * specification that defines the ATT error.
+     */
     uint16_t status;
 
     /** The attribute handle associated with the error. */