docs: attempt documenting exported modules

this doesn't actually show up when you import these things. looking into
it. in the meantime, it's useful to have the notes I wrote, so we can
actually export them in the future.

src/index.ts

@@ -16,11 +16,33 @@ export * as Friendbot from './friendbot';
 // Horizon-related classes to expose
 export * as Horizon from './horizon';
 
-// Soroban RPC-related classes to expose
+/**
+ * Tools for interacting with the Soroban RPC server, such as `Server`,
+ * `assembleTransaction`, and the `Api` types. You can import these from the
+ * `/rpc` entrypoint, if your version of Node and your TypeScript configuration
+ * allow it:
+ *
+ * ```ts
+ * import { Server } from '@stellar/stellar-sdk/rpc';
+ * ```
+ */
 export * as rpc from './rpc';
+
+/**
+ * @deprecated Use `rpc` instead
+ */
 export * as SorobanRpc from './rpc';
 
-// Soroban Contract-related classes to expose
+/**
+ * Tools for interacting with smart contracts, such as `Client`, `Spec`, and
+ * `AssembledTransaction`. You can import these from the `/contract`
+ * entrypoint, if your version of Node and your TypeScript configuration allow
+ * it:
+ *
+ * ```ts
+ * import { Client } from '@stellar/stellar-sdk/contract';
+ * ```
+ */
 export * as contract from './contract'
 
 // expose classes and functions from stellar-base