clarify retransmit-related fields of TcpInfo

Based on https://lore.kernel.org/netdev/CADVnQyn=6wYbVP0m3mepGU23LcEn_BK_TKoSNxVf=TUz9Q+f8g@mail.gmail.com/

Additionally, tcpi_retransmits was reworded for broader audience; I believe
new description is correct based on where icsk_retransmits (source of
tcpi_retransmits) get reset to 0 in the kernel:

- tcp_connect_init()
- tcp_ack()
- tcp_try_undo_loss()
- tcp_rcv_synrecv_state_fastopen()

Author: vthriller

src/inet/nlas.rs

@@ -476,7 +476,7 @@ pub struct TcpInfo {
     /// `TCP_CA_DISORDER`, `TCP_CA_CWR`, `TCP_CA_RECOVERY`,
     /// `TCP_CA_LOSS`
     pub ca_state: u8,
-    ///
+    /// Number of retransmits since the last ACK
     pub retransmits: u8,
     pub probes: u8,
     pub backoff: u8,
@@ -506,7 +506,7 @@ pub struct TcpInfo {
     pub sacked: u32,
     /// Number of segments that have been lost
     pub lost: u32,
-    /// Number of segments that have been retransmitted
+    /// Number of segments that are currently being retransmitted
     pub retrans: u32,
     /// Number of segments that have been FACKed
     pub fackets: u32,
@@ -538,6 +538,7 @@ pub struct TcpInfo {
     pub rcv_rtt: u32,
     pub rcv_space: u32,
 
+    /// Number of segments that have been retransmitted during lifetime of the socket
     pub total_retrans: u32,
 
     pub pacing_rate: u64,