process: clarify Child::kill behavior (#7162)

Author: stepancheg

Diff for: tokio/src/process/mod.rs

@@ -1164,7 +1164,12 @@ impl Child {
 
     /// Forces the child to exit.
     ///
-    /// This is equivalent to sending a `SIGKILL` on unix platforms.
+    /// This is equivalent to sending a `SIGKILL` on unix platforms
+    /// followed by [`wait`](Child::wait).
+    ///
+    /// Note: std version of [`Child::kill`](std::process::Child::kill) does not `wait`.
+    /// For an equivalent of `Child::kill` in the standard library,
+    /// use [`start_kill`](Child::start_kill).
     ///
     /// # Examples
     ///