docs: fix incorrect feature name references in gix-transport

[ArcticLampyrid (ArcticLampyrid)]

Summary

This PR fixes documentation issues in gix-transport/Cargo.toml related to the reqwest feature names:

1. Incorrect feature name references: Replaced blocking-http-transport-reqwest with the correct http-client-reqwest in the feature documentation comments. These incorrect references were carried over from a March 2024 PR (Remove dep reqwest from gix #1329) that migrated the rustls feature into gix-transport.

2. Clarified HTTPS support: Added a note to the http-client-reqwest feature documentation that HTTPS is NOT supported by default. Users must enable one of:

   • http-client-reqwest-rust-tls
   • http-client-reqwest-native-tls

   This addresses user confusion where HTTPS requests would fail with an unhelpful error message when only http-client-reqwest was enabled.

Related

Fixes #2425

Changes

-## Implies `http-client` and adds support for http and https transports using the blocking version of `reqwest`.
+## Implies `http-client` and adds support for http transports using the blocking version of `reqwest`.
+## NOTE: `https://` is NOT supported by default. You must enable one of the `http-client-reqwest-rust-tls`
+## or `http-client-reqwest-native-tls` features to enable HTTPS support.
 http-client-reqwest = ["reqwest", "http-client"]
-## Stacks with `blocking-http-transport-reqwest` and enables `https://` via the `rustls` crate.
+## Stacks with `http-client-reqwest` and enables `https://` via the `rustls` crate.

[Sebastian Thiel (Byron)]

Thanks a lot, much appreciated.