docs: Improve dir/dirs/nodir migration guide

Author: cako

MIGRATION_V1_V2.md

@@ -8,7 +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`.
+- `dir`, `dirs` and `nodir` have been deprecated in favor of `axis` and `axes`. When previously `nodir` was required, you must now provide the directions along which the operator is applied in `axes`. The default value for `axis` and `axes` are chosen to be -1 and (-2, -1), respectively, whereas the default `dir` and `dirs` was -1 and (0, 1), respectively. Be careful to check all operators where `dir`, `dirs` and `nodir` was not provided explicitly.
 - `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`.