buffer: docs-only deprecate SlowBuffer

With the addition of `Buffer.allocUnsafeSlow(size)`
`SlowBuffer` can be deprecated... but docs-only for now.

PR-URL: #5833
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>

doc/api/buffer.markdown

@@ -1783,6 +1783,9 @@ Note that this is a property on the `buffer` module as returned by
 
 ## Class: SlowBuffer
 
+    Stability: 0 - Deprecated: Use
+    [`Buffer.allocUnsafeSlow(size)`][buffer_allocunsafeslow] instead.
+
 Returns an un-pooled `Buffer`.
 
 In order to avoid the garbage collection overhead of creating many individually
@@ -1813,6 +1816,9 @@ has observed undue memory retention in their applications.
 
 ### new SlowBuffer(size)
 
+    Stability: 0 - Deprecated: Use
+    [`Buffer.allocUnsafeSlow(size)`][buffer_allocunsafeslow] instead.
+
 * `size` Number
 
 Allocates a new `SlowBuffer` of `size` bytes.  The `size` must be less than