Incorrect comment for CURL_HTTP_VERSION_3

[jcamiel]

To use HTTP 3 we can use the following code:

self.handle.http_version(HttpVersion::V3);

HttpVersion::V3 is defined as as

curl-rust/src/easy/handler.rs

Lines 455 to 465 in ff6ad21

	/// Setting this value will make libcurl attempt to use HTTP/3 directly to
	/// server given in the URL. Note that this cannot gracefully downgrade to
	/// earlier HTTP version if the server doesn't support HTTP/3.
	///
	/// For more reliably upgrading to HTTP/3, set the preferred version to
	/// something lower and let the server announce its HTTP/3 support via
	/// Alt-Svc:.
	///
	/// (Added in CURL 7.66.0)
	V3 = curl_sys::CURL_HTTP_VERSION_3 as isize,
	}

And curl_sys::CURL_HTTP_VERSION_3 is defined here:

curl-rust/curl-sys/lib.rs

Lines 663 to 665 in ff6ad21

	/// Makes use of explicit HTTP/3 without fallback.
	/// (Added in CURL 7.66.0)
	pub const CURL_HTTP_VERSION_3: c_int = 30;

The comment of this flag states that

Makes use of explicit HTTP/3 without fallback.

Whereas in curl.h the corresponding flag is defined

https://github.com/curl/curl/blob/9bca45dba81d77de06757ce13fc9193e09e010b5/include/curl/curl.h#L2272-L2274

/* Use HTTP/3, fallback to HTTP/2 or HTTP/1 if
needed. For HTTPS only. For HTTP, this option
makes libcurl return error. */

libcurl seems to fallback when using this flag whereas comments in crates curl imply that there is no fallback...

[sagebind]

Our doc comment was lifted straight from the libcurl man page, but it looks like it actually changed behaviors in version 7.88 when CURL_HTTP_VERSION_3ONLY was added: curl/curl@a56d2b0. When we initially added this, CURL_HTTP_VERSION_3 didn't allow any fallback.

But we can update this now. Maybe worth noting in the doc comment that the behavior varies between version >=7.66,<7.88 and >=7.88, depending on which version of libcurl you are linked to.

[jcamiel]

I can propose a PR with an updated comment if that help,

[jcamiel]

PR here => #521 (relevant curl PR is curl/curl#10264)