KNOWN_BUGS: remove five FTP related issues

- "FTP with CONNECT and slow server"

I believe this is not a problem these days.

- "FTP with NULs in URL parts"

The FTP protocol does not support them properly anyway.

- remove "FTP and empty path parts in the URL"

I don't think this has ever been reported as a real problem but was only
a hypothetical one.

- "Premature transfer end but healthy control channel"

This is not a bug, this is an optimization that *could* be performed but is
not an actual problem.

- "FTP without or slow 220 response"

Instead add to the documentation of the connect timeout that the
connection is considered complete at TCP/TLS/QUIC layer.

Closes #9979

docs/libcurl/opts/CURLOPT_CONNECTTIMEOUT.3

@@ -43,6 +43,9 @@ In unix-like systems, this might cause signals to be used unless
 
 If both \fICURLOPT_CONNECTTIMEOUT(3)\fP and \fICURLOPT_CONNECTTIMEOUT_MS(3)\fP
 are set, the value set last will be used.
+
+The "connection phase" is considered complete when the requested TCP, TLS or
+QUIC handshakes are done.
 .SH DEFAULT
 300
 .SH PROTOCOLS

docs/libcurl/opts/CURLOPT_CONNECTTIMEOUT_MS.3

@@ -44,6 +44,9 @@ In unix-like systems, this might cause signals to be used unless
 
 If both \fICURLOPT_CONNECTTIMEOUT(3)\fP and \fICURLOPT_CONNECTTIMEOUT_MS(3)\fP
 are set, the value set last will be used.
+
+The "connection phase" is considered complete when the requested TCP, TLS or
+QUIC handshakes are done.
 .SH DEFAULT
 300000
 .SH PROTOCOLS