doc/next: improve description of proposal 61308

For #61308.
For #65614.

Change-Id: I36b4f2392075d5a3fb9f53a28bd19b997e7be363
Reviewed-on: https://go-review.googlesource.com/c/go/+/592197
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Bypass: Dmitri Shuralyov <dmitshur@golang.org>

Author: dmitshur

doc/next/6-stdlib/99-minor/reflect/61308.md

@@ -1,3 +1,3 @@
-The [SliceAt(typ Type, p unsafe.Pointer, len int)] function
-returns a Value representing a slice whose underlying array starts
-at p and whose length and capacity are len.
+The new [SliceAt] function is analogous to [NewAt], but for slices.
+
+The [Value.Pointer] and [Value.UnsafePointer] methods now support values of kind [String].