docs: lead with the protocol coverage, and drop a stale upload assertion

The message-set coverage is the crate's strongest claim, so say it where
people look first: the README features list and About, the website home
and community pages, each pointing at the coverage table. The table now
carries a section naming what the message set does not reach, so the
claim has something behind it rather than standing on its own.

The upload streaming test still asserted the uploader was blocked
waiting for the downloader's close — the wait that came out when it
started costing the upload slot. It passed only while the thread had yet
to be scheduled, and failed whenever it had.

soulseek-rs-lib/src/peer/upload_peer.rs

@@ -189,10 +189,6 @@ mod tests {
         stream.read_exact(&mut received).unwrap();
         assert_eq!(received, content);
 
-        assert!(
-            !uploader.is_finished(),
-            "the uploader must wait for the downloader to close the F socket"
-        );
         drop(stream);
         assert_eq!(uploader.join().unwrap().unwrap(), 4096);
         assert_eq!(bytes_sent.load(Ordering::Relaxed), 4096);