[Containers] document custom instance types in configuration.mdx

src/content/docs/workers/wrangler/configuration.mdx

@@ -1133,6 +1133,37 @@ new_sqlite_classes = ["MyContainer"]
 
 </WranglerConfig>
 
+### Custom Instance Types
+
+For Enterprise customers only.
+
+In place of the [named instance types](/containers/platform-details#instance-types), you can set a custom instance type by
+individually configuring vCPU, memory, and disk.
+
+The following options are available:
+
+- `vcpu` <Type text="number" /> <MetaInfo text="optional" />
+
+  - The vCPU to be used by your container. Defaults to 1/16 vCPU.
+
+- `memory_mib` <Type text="number" /> <MetaInfo text="optional" />
+
+  - The memory to be used by your container, in MiB. Defaults to 256 MiB.
+
+- `disk_mb` <Type text="number" /> <MetaInfo text="optional" />
+
+  - The disk to be used by your container, in MB. Defaults to 2GB.
+
+<WranglerConfig>
+
+```toml title="wrangler.toml"
+[[containers]]
+image = "./Dockerfile"
+instance_type = { vcpu = 1, memory_mib = 1024, disk_mb = 4000 }
+```
+
+</WranglerConfig>
+
 ## Bundling
 
 :::note