Add solana-cli-config link to rust-api.md (backport #21840)

docs/src/developing/clients/rust-api.md

@@ -21,11 +21,16 @@ Some important crates:
 - [`solana-client`] — For interacting with a Solana node via the
   [JSON RPC API](jsonrpc-api).
 
+- [`solana-cli-config`] — Loading and saving the Solana CLI configuration
+  file.
+
 - [`solana-clap-utils`] — Routines for setting up a CLI, using [`clap`],
-  as used by the main Solana CLI.
+  as used by the main Solana CLI. Includes functions for loading all types of
+  signers supported by the CLI.
 
 [`solana-program`]: https://docs.rs/solana-program
 [`solana-sdk`]: https://docs.rs/solana-sdk
 [`solana-client`]: https://docs.rs/solana-client
+[`solana-cli-config`]: https://docs.rs/solana-cli-config
 [`solana-clap-utils`]: https://docs.rs/solana-clap-utils
 [`clap`]: https://docs.rs/clap