doc: migration-guide-4.3: Add entry about file descriptor table changes

The file descriptor table size is now controlled by adding together
the values of CONFIG_ZVFS_OPEN_ADD_SIZE_* options.

Author: jukkar

doc/releases/migration-guide-4.3.rst

@@ -41,6 +41,15 @@ Base Libraries
 * ``Z_MIN``, ``Z_MAX`` and ``Z_CLAMP`` macros have been renamed to
   :c:macro:`min` :c:macro:`max` and :c:macro:`clamp`.
 
+* The number of file descriptor table size and its availability is now determined by
+  a ``ZVFS_OPEN_SIZE`` define instead of the :kconfig:option:`CONFIG_ZVFS_OPEN_MAX`
+  Kconfig option. Subsystems can specify their own custom file descriptor table size
+  requirements by specifying Kconfig options with the prefix ``CONFIG_ZVFS_OPEN_ADD_SIZE_``.
+  The old Kconfig option still exists, but will be overridden if the custom requirements
+  are larger. To force the old Kconfig option to be used, even when its value is less
+  than the indicated custom requirements, a new :kconfig:option:`CONFIG_ZVFS_OPEN_IGNORE_MIN`
+  option has been introduced (which defaults being disabled).
+
 Boards
 ******