feat: Add ability to disable connection-pooling behavior for Dynamic Backends (#574)

Author: JakeChampion

documentation/docs/fastly:backend/Backend/Backend.mdx

@@ -63,6 +63,12 @@ new Backend(backendConfiguration)
       - Throws a [`RangeError`](../../globals/RangeError/RangeError.mdx) if the value is negative or greater than or equal to 2^32
     - `useSSL` _: boolean_ _**optional**_
       - Whether or not to require TLS for connections to this backend.
+    - `dontPool` _: boolean_ _**optional**_
+      - Determine whether or not connections to the same backend should be pooled across different sessions.
+      - Fastly considers two backends “the same” if they're registered with the same name and the exact same settings.
+      - In those cases, when pooling is enabled, if Session 1 opens a connection to this backend it will be left open, and can be re-used by Session 2.
+      - This can help improve backend latency, by removing the need for the initial network / TLS handshake(s).
+      - By default, pooling is enabled for dynamic backends.
     - `tlsMinVersion` _: 1 | 1.1 | 1.2 | 1.3_ _**optional**_
       - Minimum allowed TLS version on SSL connections to this backend.
       - If the backend server is not able to negotiate a connection meeting this constraint, a 503 response will be presented instead.