Rollup merge of rust-lang#130743 - YohDeadfall:net-nonblocking-doc, r…

…=Mark-Simulacrum

Clarifications for set_nonblocking methods

Closes rust-lang#129903.

The issue mentions that `send`, `recv` and other operations are interpreted by some users as methods of `TcpSocket` which led to confusion since it hasn't them. To fix it I added "system" into the documentation as being more precise for two reasons:
* it's makes it clear that these names are system operations;
* it doesn't point to the location of these methods like `libc` because not every system is POSIX compatible.

std/src/net/tcp.rs

@@ -561,7 +561,7 @@ impl TcpStream {
 
     /// Moves this TCP stream into or out of nonblocking mode.
     ///
-    /// This will result in `read`, `write`, `recv` and `send` operations
+    /// This will result in `read`, `write`, `recv` and `send` system operations
     /// becoming nonblocking, i.e., immediately returning from their calls.
     /// If the IO operation is successful, `Ok` is returned and no further
     /// action is required. If the IO operation could not be completed and needs

std/src/net/udp.rs

@@ -764,7 +764,7 @@ impl UdpSocket {
 
     /// Moves this UDP socket into or out of nonblocking mode.
     ///
-    /// This will result in `recv`, `recv_from`, `send`, and `send_to`
+    /// This will result in `recv`, `recv_from`, `send`, and `send_to` system
     /// operations becoming nonblocking, i.e., immediately returning from their
     /// calls. If the IO operation is successful, `Ok` is returned and no
     /// further action is required. If the IO operation could not be completed