fix: mention pgsslmode in the docs for external db (#900)

This is particularly for the helm chart, since a customer was asking
about this. Docs applied from @marcleblanc2's recommendation in the
[linear
thread](https://linear.app/sourcegraph/issue/REL-638/configure-aws-rds-databases-for-tls-connections-in-helm-chart)

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Author: Chickensoupwithrice

docs/admin/deploy/kubernetes/index.mdx

@@ -103,6 +103,7 @@ data:
   password: ""
   port: ""
   user: ""
+  pgsslmode: "require" # optional, enable if using SSL
 ---
 apiVersion: v1
 kind: Secret
@@ -115,6 +116,7 @@ data:
   password: ""
   port: ""
   user: ""
+  pgsslmode: "require" # optional, enable if using SSL
 ---
 apiVersion: v1
 kind: Secret
@@ -127,6 +129,7 @@ data:
   password: ""
   port: ""
   user: ""
+  pgsslmode: "require" # optional, enable if using SSL
 ```
 
 The above Secrets should be deployed to the same namespace as the existing Sourcegraph deployment.
@@ -163,6 +166,7 @@ pgsql:
     user: "new-user"
     password: "new-password"
     port: "5432"
+    pgsslmode: "require" # optional, enable if using SSL
 ```
 
 #### Using external Redis instances