Auto merge of #40829 - mgattozzi:ChildStderr, r=steveklabnik

Update ChildStderr docs to be clearer

Before the docs only had a line about where it was found and that it was
a handle to stderr. This commit changes it so that the summary second line is
removed and that it's a bit clearer about what can be done with it. Part of
#29370

src/libstd/process.rs

@@ -233,8 +233,9 @@ fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
     }
 }
 
-/// A handle to a child process's stderr. This struct is used in the [`stderr`]
-/// field on [`Child`].
+/// A handle to a child process's stderr.
+///
+/// This struct is used in the [`stderr`] field on [`Child`].
 ///
 /// [`Child`]: struct.Child.html
 /// [`stderr`]: struct.Child.html#structfield.stderr