feat(error): Error::source() is purposefully unspecified (#3318)

Document that the exact types returned in the erased errors of
`Error::source()` may change at any moment, and cannot be depended on.

Closes #2843

BREAKING CHANGE: Do not build any logic depending on the exact types of
  an `Error::source()`. They are only for debugging.

src/error.rs

@@ -20,6 +20,14 @@ type Cause = Box<dyn StdError + Send + Sync>;
 /// The contents of the formatted error message of this specific `Error` type
 /// is unspecified. **You must not depend on it.** The wording and details may
 /// change in any version, with the goal of improving error messages.
+///
+/// # Source
+///
+/// A `hyper::Error` may be caused by another error. To aid in debugging,
+/// those are exposed in `Error::source()` as erased types. While it is
+/// possible to check the exact type of the sources, they **can not be depended
+/// on**. They may come from private internal dependencies, and are subject to
+/// change at any moment.
 pub struct Error {
     inner: Box<ErrorImpl>,
 }