doc: extended doc.h concerning errors
/**
* @brief Sets the error in the keys metadata.
*
- * Include kdberrors.h to make it work:
+ * Include kdberrors.h to make it work.
+ * Only a single error can be written to the key.
*
* @snippet doc.c plugin errors include
*
/**
* @brief Sets the error in the keys metadata.
*
- * Include kdberrors.h to make it work:
+ * Include kdberrors.h to make it work.
+ * Only a single error can be written to the key.
*
* @snippet doc.c plugin errors include
*
