Expand on doc comments a bit

src/lib.rs

@@ -86,7 +86,10 @@ impl<T: Copy + Default + Step, const STEP: usize> Iterator for DecBy<T, STEP> {
 
 /// A subtrait of [`RangeBounds<T>`](core::ops::RangeBounds) where `T` is `Copy + Default + Step`
 /// that turns implementers of it into an instance of [`IncBy`][crate::IncBy] when
-/// [`inc_by`](crate::IntoIncBy::inc_by) is called.
+/// [`inc_by`](crate::IntoIncBy::inc_by) is called. Currently, the blanket implementation of this
+/// trait exported from the crate for `RangeBounds` is the only possible useful implementation, but
+/// it was decided not to make it a `Sealed` trait in case additional methods are added in the
+/// future that would be implementable by end-user code in a meaningfully varied way.
 pub trait IntoIncBy<T: Copy + Default + Step>: RangeBounds<T> {
   /// Functionally equivalent to what [`step_by`](core::iter::Iterator::step_by) does when it is
   /// called through a primitive range, but written specifically with primitive ranges in mind such
@@ -110,7 +113,10 @@ pub trait IntoIncBy<T: Copy + Default + Step>: RangeBounds<T> {
 
 /// A subtrait of [`RangeBounds<T>`](core::ops::RangeBounds) where `T` is `Copy + Default + Step`
 /// that turns implementers of it into an instance of [`DecBy`][crate::DecBy] when
-/// [`dec_by`](crate::IntoDecBy::dec_by) is called.
+/// [`dec_by`](crate::IntoDecBy::dec_by) is called. Currently, the blanket implementation of this
+/// trait exported from the crate for `RangeBounds` is the only possible useful implementation, but
+/// it was decided not to make it a `Sealed` trait in case additional methods are added in the
+/// future that would be implementable by end-user code in a meaningfully varied way.
 pub trait IntoDecBy<T: Copy + Default + Step>: RangeBounds<T> {
   /// Functionally equivalent to what [`step_by`](core::iter::Iterator::step_by) does when it is
   /// called through a primitive range, but specifically in reverse and operating directly on