bus/spi: add semver risk warning to new_no_delay.

Fixes rust-embedded#552.

embedded-hal-bus/src/spi/critical_section.rs

@@ -33,6 +33,16 @@ impl<'a, BUS, CS, D> CriticalSectionDevice<'a, BUS, CS, D> {
 
 impl<'a, BUS, CS> CriticalSectionDevice<'a, BUS, CS, super::NoDelay> {
     /// Create a new [`CriticalSectionDevice`] without support for in-transaction delays.
+    /// 
+    /// **Warning**: The returned instance *technically* doesn't comply with the `SpiDevice`
+    /// contract, which mandates delay support. It is relatively rare for drivers to use
+    /// in-transaction delays, so you might still want to use this method because it's more practical.
+    /// 
+    /// Note that a future version of the driver might start using delays, causing your
+    /// code to panic. This wouldn't be considered a breaking change from the driver side, because
+    /// drivers are allowed to assume `SpiDevice` implementations comply with the contract. 
+    /// If you feel this risk outweighs the convenience of having `cargo` automatically upgrade
+    /// the driver crate, you might want to pin the driver's version.
     ///
     /// # Panics
     ///

embedded-hal-bus/src/spi/exclusive.rs

@@ -44,6 +44,16 @@ impl<BUS, CS, D> ExclusiveDevice<BUS, CS, D> {
 
 impl<BUS, CS> ExclusiveDevice<BUS, CS, super::NoDelay> {
     /// Create a new [`ExclusiveDevice`] without support for in-transaction delays.
+    /// 
+    /// **Warning**: The returned instance *technically* doesn't comply with the `SpiDevice`
+    /// contract, which mandates delay support. It is relatively rare for drivers to use
+    /// in-transaction delays, so you might still want to use this method because it's more practical.
+    /// 
+    /// Note that a future version of the driver might start using delays, causing your
+    /// code to panic. This wouldn't be considered a breaking change from the driver side, because
+    /// drivers are allowed to assume `SpiDevice` implementations comply with the contract. 
+    /// If you feel this risk outweighs the convenience of having `cargo` automatically upgrade
+    /// the driver crate, you might want to pin the driver's version.
     ///
     /// # Panics
     ///

embedded-hal-bus/src/spi/mutex.rs

@@ -31,6 +31,16 @@ impl<'a, BUS, CS, D> MutexDevice<'a, BUS, CS, D> {
 
 impl<'a, BUS, CS> MutexDevice<'a, BUS, CS, super::NoDelay> {
     /// Create a new [`MutexDevice`] without support for in-transaction delays.
+    /// 
+    /// **Warning**: The returned instance *technically* doesn't comply with the `SpiDevice`
+    /// contract, which mandates delay support. It is relatively rare for drivers to use
+    /// in-transaction delays, so you might still want to use this method because it's more practical.
+    /// 
+    /// Note that a future version of the driver might start using delays, causing your
+    /// code to panic. This wouldn't be considered a breaking change from the driver side, because
+    /// drivers are allowed to assume `SpiDevice` implementations comply with the contract. 
+    /// If you feel this risk outweighs the convenience of having `cargo` automatically upgrade
+    /// the driver crate, you might want to pin the driver's version.
     ///
     /// # Panics
     ///

embedded-hal-bus/src/spi/refcell.rs

@@ -30,6 +30,16 @@ impl<'a, BUS, CS, D> RefCellDevice<'a, BUS, CS, D> {
 
 impl<'a, BUS, CS> RefCellDevice<'a, BUS, CS, super::NoDelay> {
     /// Create a new [`RefCellDevice`] without support for in-transaction delays.
+    /// 
+    /// **Warning**: The returned instance *technically* doesn't comply with the `SpiDevice`
+    /// contract, which mandates delay support. It is relatively rare for drivers to use
+    /// in-transaction delays, so you might still want to use this method because it's more practical.
+    /// 
+    /// Note that a future version of the driver might start using delays, causing your
+    /// code to panic. This wouldn't be considered a breaking change from the driver side, because
+    /// drivers are allowed to assume `SpiDevice` implementations comply with the contract. 
+    /// If you feel this risk outweighs the convenience of having `cargo` automatically upgrade
+    /// the driver crate, you might want to pin the driver's version.
     ///
     /// # Panics
     ///