Add warning about initial_node_count (#3851) (#2363)

Upstreamed this change from hashicorp/terraform-provider-google#6896

Signed-off-by: Modular Magician <[email protected]>

Author: modular-magician

.changelog/3851.txt

@@ -0,0 +1,2 @@
+```release-note:none
+```

website/docs/r/container_node_pool.html.markdown

@@ -113,8 +113,12 @@ resource "google_container_cluster" "primary" {
     the size of the node pool to the current cluster usage. Structure is documented below.
 
 * `initial_node_count` - (Optional) The initial number of nodes for the pool. In
-regional or multi-zonal clusters, this is the number of nodes per zone. Changing
-this will force recreation of the resource.
+    regional or multi-zonal clusters, this is the number of nodes per zone. Changing
+    this will force recreation of the resource. WARNING: Resizing your node pool manually
+    may change this value in your existing cluster, which will trigger destruction
+    and recreation on the next Terraform run (to rectify the discrepancy).  If you don't
+    need this value, don't set it.  If you do need it, you can [use a lifecycle block to
+    ignore subsqeuent changes to this field](https://github.com/hashicorp/terraform-provider-google/issues/6901#issuecomment-667369691).
 
 * `management` - (Optional) Node management configuration, wherein auto-repair and
     auto-upgrade is configured. Structure is documented below.