Merge #129: doc: fix cargo doc builds when non-default features selected

6eece87 doc: fix cargo doc builds when non-default features selected (Philip Kannegaard Hayes)

Pull request description:

  While upgrading to `v0.12.0` our `cargo doc` builds broke:

  ```bash
  $ cargo doc --no-default-features --features=async,tokio

  # ...

  error: unresolved link to `minreq`
    --> /Users/phlip9/dev/esplora-client/src/lib.rs:54:42
     |
  54 | //! * `blocking-https-bundled` enables [`minreq`], the blocking client with proxy and TLS (SSL)
     |                                          ^^^^^^ no item named `minreq` in scope
     |
     = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`

  error: public documentation for `max_retries` links to private item `RETRYABLE_ERROR_CODES`
     --> /Users/phlip9/dev/esplora-client/src/lib.rs:171:21
      |
  171 |     /// is one of [`RETRYABLE_ERROR_CODES`].
      |                     ^^^^^^^^^^^^^^^^^^^^^ this item is private

  # ... etc
  ```

  To keep the nice doc links to `minreq` and `reqwest`, I just pointed them to <https://docs.rs/minreq> and <https://docs.rs/reqwest> respectively. It won't resolve to the exact version, but it's better than nothing IMO.

ACKs for top commit:
  notmandatory:
    ACK 6eece87

Tree-SHA512: fafe24422f41c0360f2138af0ea133c98bf88fcd5b4012a1cf336af51cfb309cfc3f867ce3c061e03519560ac2e90fe20d3b50accaf1bbbfafdfd966c12f1e21

Author: notmandatory

src/lib.rs

@@ -63,7 +63,10 @@
 //! * `async-https-rustls-manual-roots` enables [`reqwest`], the async client with support for
 //!   proxying and TLS (SSL) using the `rustls` TLS backend without using its the default root
 //!   certificates.
-
+//!
+//! [`dont remove this line or cargo doc will break`]: https://example.com
+#![cfg_attr(not(feature = "minreq"), doc = "[`minreq`]: https://docs.rs/minreq")]
+#![cfg_attr(not(feature = "reqwest"), doc = "[`reqwest`]: https://docs.rs/reqwest")]
 #![allow(clippy::result_large_err)]
 
 use std::collections::HashMap;
@@ -87,7 +90,7 @@ pub use blocking::BlockingClient;
 pub use r#async::AsyncClient;
 
 /// Response status codes for which the request may be retried.
-const RETRYABLE_ERROR_CODES: [u16; 3] = [
+pub const RETRYABLE_ERROR_CODES: [u16; 3] = [
     429, // TOO_MANY_REQUESTS
     500, // INTERNAL_SERVER_ERROR
     503, // SERVICE_UNAVAILABLE