update body type migration guide

actix-http/src/body/either.rs

@@ -10,6 +10,17 @@ use super::{BodySize, BoxBody, MessageBody};
 use crate::Error;
 
 pin_project! {
+    /// An "either" type specialized for body types.
+    ///
+    /// It is common, in middleware especially, to conditionally return an inner service's unknown/
+    /// generic body `B` type or return early with a new response. This type's "right" variant
+    /// defaults to `BoxBody` since error responses are the common case.
+    ///
+    /// For example, middleware will often have `type Response = ServiceResponse<EitherBody<B>>`.
+    /// This means that the inner service's response body type maps to the `Left` variant and the
+    /// middleware's own error responses use the default `Right` variant of `BoxBody`. Of course,
+    /// there's no reason it couldn't use `EitherBody<B, String>` instead if its alternative
+    /// responses have a known type.
     #[project = EitherBodyProj]
     #[derive(Debug, Clone)]
     pub enum EitherBody<L, R = BoxBody> {
@@ -22,7 +33,10 @@ pin_project! {
 }
 
 impl<L> EitherBody<L, BoxBody> {
-    /// Creates new `EitherBody` using left variant and boxed right variant.
+    /// Creates new `EitherBody` left variant with a boxed right variant.
+    ///
+    /// If the expected `R` type will be inferred and is not `BoxBody` then use the
+    /// [`left`](Self::left) constructor instead.
     #[inline]
     pub fn new(body: L) -> Self {
         Self::Left { body }