gopls/doc/release/v0.21.0.md: restore (deleted in CL 691115)

Change-Id: Id5ad7517c8747d25fc622c99194103c3bd6dcea0
Reviewed-on: https://go-review.googlesource.com/c/tools/+/691375
Auto-Submit: Alan Donovan <adonovan@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: adonovan

gopls/doc/release/v0.21.0.md

@@ -0,0 +1,21 @@
+---
+title: "Gopls release v0.21.0 (forthcoming)"
+---
+
+## Configuration changes
+## Web-based features
+## Editing features
+## Analysis features
+## Code transformation features
+
+<!-- golang/go#42301 -->
+The Rename operation now treats [Doc Links](https://tip.golang.org/doc/comment#doclinks)
+like identifiers, so you can initiate a renaming from a Doc Link.
+
+<!--
+
+### $feature
+
+https://go.dev/issue#xxxxx
+
+-->