doc/next: add release note for sync/Map.Clear

Fix https://go-review.git.corp.google.com/c/go/+/561456

For #61696

Change-Id: I573bd14cf0e5c41adcc8c2b9481e85c5792c2a82
GitHub-Last-Rev: c6fce6d
GitHub-Pull-Request: #65534
Reviewed-on: https://go-review.googlesource.com/c/go/+/561755
Reviewed-by: David Chase <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Jonathan Amsterdam <[email protected]>

Author: Nasfame

doc/next/6-stdlib/99-minor/sync/61696.md

@@ -1 +1,4 @@
-TODO([email protected])
+The [`(*sync.Map) Clear()`](//sync#Map.Clear) method deletes
+all the entries, resulting in an empty map
+([#61696](https://github.com/golang/go/issues/61696)).
+It is analogous to `clear`.