Add v20 changelog docs for PR #15030 (#15367)

Signed-off-by: Tim Vaillancourt <tim@timvaillancourt.com>
Signed-off-by: Florent Poinsard <florent.poinsard@outlook.fr>
Co-authored-by: Florent Poinsard <florent.poinsard@outlook.fr>

changelog/20.0/20.0.0/summary.md

@@ -16,6 +16,7 @@
 - **[Minor Changes](#minor-changes)**
   - **[New Stats](#new-stats)**
     - [VTTablet Query Cache Hits and Misses](#vttablet-query-cache-hits-and-misses)
+  - **[`SIGHUP` reload of gRPC client static auth creds](#sighup-reload-of-grpc-client-auth-creds)**
 
 ## <a id="major-changes"/>Major Changes
 
@@ -87,3 +88,7 @@ VTTablet exposes two new counter stats:
 
  * `QueryCacheHits`: Query engine query cache hits
  * `QueryCacheMisses`: Query engine query cache misses
+
+### <a id="sighup-reload-of-grpc-client-auth-creds"/>`SIGHUP` reload of gRPC client static auth creds
+
+The internal gRPC client now caches the static auth credentials and supports reloading via the `SIGHUP` signal. Previous to v20 the credentials were not cached. They were re-loaded from disk on every use.