docs: Document dir to axis migration

Author: cako

MIGRATION_V1_V2.md

@@ -8,6 +8,7 @@ should be used as a checklist when converting a piece of code using PyLops from
 - Several operators have deprecated `N` as a keyword. To migrate, pass only `dims` if both `N` and `dims` are currently
   being passed. If only `N` is being passed, ensure it is being passed as a value and not a keyword argument (e.g.,
   change `Flip(N=100)` to `Flip(100)`).
+- `dir`, `dirs` and `nodir` have been deprecated in favor of `axis` and `axes`.
 - `utils.dottest`: Change `tol` into `rtol`. Absolute tolerance is now also supported via the keyword `atol`.
   When calling it with purely positional arguments, note that after `rtol` comes now first `atol` before `complexflag`.
   When using `raiseerror=True` it now emits an `AttributeError` instead of a `ValueError`.