rust: allow `clippy::disallowed_names` for doctests

rust: allow `clippy::disallowed_names` for doctests

Examples (i.e. doctests) may want to use names such as `foo`, thus the
`clippy::disallowed_names` lint [1] gets in the way.

Thus allow it for all doctests.

In addition, remove it from the existing `expect`s we have in a few
doctests.

This does not mean that we should stop trying to find good names for
our examples, though.

Link: https://rust-lang.github.io/rust-clippy/stable/index.html#disallowed_names
Suggested-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/rust-for-linux/aRHSLChi5HYXW4-9@google.com/
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Acked-by: Benno Lossin <lossin@kernel.org>
Link: https://patch.msgid.link/20251117080714.876978-1-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>