clarify doc for Iterators.reverse (#43522)

Author: stevengj

base/iterators.jl

@@ -72,15 +72,20 @@ and_iteratoreltype(a, b) = EltypeUnknown()
 
 Given an iterator `itr`, then `reverse(itr)` is an iterator over the
 same collection but in the reverse order.
-
 This iterator is "lazy" in that it does not make a copy of the collection in
 order to reverse it; see [`Base.reverse`](@ref) for an eager implementation.
 
+(By default, this returns
+an `Iterators.Reverse` object wrapping `itr`, which is iterable
+if the corresponding [`iterate`](@ref) methods are defined, but some `itr` types
+may implement more specialized `Iterators.reverse` behaviors.)
+
 Not all iterator types `T` support reverse-order iteration.  If `T`
 doesn't, then iterating over `Iterators.reverse(itr::T)` will throw a [`MethodError`](@ref)
-because of the missing [`iterate`](@ref) methods for `Iterators.Reverse{T}`.
+because of the missing `iterate` methods for `Iterators.Reverse{T}`.
 (To implement these methods, the original iterator
-`itr::T` can be obtained from `r = Iterators.reverse(itr)` by `r.itr`.)
+`itr::T` can be obtained from an `r::Iterators.Reverse{T}` object by `r.itr`;
+more generally, one can use `Iterators.reverse(r)`.)
 
 # Examples
 ```jldoctest