transition DOC add paragraph about validation flags
* libyang 1.x, adding the default nodes was made available as a standalone function to simplify the data manipulation
* process before the final validation.
*
+ * Many validation flags were removed because they became obsolete (LYD_OPT_DESTRUCT, LYD_OPT_WHENAUTODEL,
+ * LYD_OPT_NOEXTDEPS, LYD_OPT_DATA_NO_YANGLIB, LYD_OPT_VAL_DIFF) or we consider them redundant (LYD_OPT_OBSOLETE,
+ * LYD_OPT_NOSIBLINGS, LYD_OPT_DATA_ADD_YANGLIB). As for LYD_OPT_TRUSTED, this option was mostly replaced with
+ * ::LYD_PARSE_ONLY because both skip validation but with the significant difference that LYD_OPT_TRUSTED also sets
+ * the data node flags to be considered validated. The current option ::LYD_PARSE_ONLY does not do this because
+ * there is now much better support for working with non-validated data trees. Also, in case an invalid tree was marked
+ * as validated, it could lead to some undesired behavior, which is now avoided.
+ *
+ * Worth mentioning is a difference in the LYB format, which now also stores its validation flags. So, in case
+ * a validated data tree is stored, it is also loaded as validated.
+ *
* libyang 1.x | libyang 2.0 | Notes
* :----------------------------|:--------------------------------|:---------------------------------------------------------
* lyd_validate() | ::lyd_validate_all(), ::lyd_validate_op() | Redesigned functionality.
