properly document let binding workaround

[BennoLossin]

The three let bindings (in the bodies of cast_init, cast_pin_init and the init! macro) are used to avoid the following compiler error in Rust 1.78.0, 1.79.0, 1.80.0, 1.80.1, and 1.81.0 (just showing the one for cast_init, the others are similar):

error[E0391]: cycle detected when computing type of opaque `cast_init::{opaque#0}`
    --> src/lib.rs:1160:66
     |
1160 | pub const unsafe fn cast_init<T, U, E>(init: impl Init<T, E>) -> impl Init<U, E> {
     |                                                                  ^^^^^^^^^^^^^^^
     |
note: ...which requires borrow-checking `cast_init`...
    --> src/lib.rs:1160:1
     |
1160 | pub const unsafe fn cast_init<T, U, E>(init: impl Init<T, E>) -> impl Init<U, E> {
     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: ...which requires const checking `cast_init`...
    --> src/lib.rs:1160:1
     |
1160 | pub const unsafe fn cast_init<T, U, E>(init: impl Init<T, E>) -> impl Init<U, E> {
     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     = note: ...which requires computing whether `cast_init::{opaque#0}` is freeze...
     = note: ...which requires evaluating trait selection obligation `cast_init::{opaque#0}: core::marker::Freeze`...
     = note: ...which again requires computing type of opaque `cast_init::{opaque#0}`, completing the cycle
note: cycle used when computing type of `cast_init::{opaque#0}`
    --> src/lib.rs:1160:66
     |
1160 | pub const unsafe fn cast_init<T, U, E>(init: impl Init<T, E>) -> impl Init<U, E> {
     |                                                                  ^^^^^^^^^^^^^^^
     = note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information

Once we raise the nightly-MSRV above 1.81, we can remove this workaround.

[BennoLossin]

@nbdd0121 happy with this?

[BennoLossin]

Ah this also conflicts with #102, we can merge that first as well.