unix_sigpipe: Add docs for init() sigpipe param

library/std/src/rt.rs

@@ -72,6 +72,8 @@ macro_rules! rtunwrap {
 // Runs before `main`.
 // SAFETY: must be called only once during runtime initialization.
 // NOTE: this is not guaranteed to run, for example when Rust code is called externally.
+// The extra parameter `sigpipe` allows rustc to generate code that instructs std whether
+// or not to ignore `SIGPIPE`.
 #[cfg_attr(test, allow(dead_code))]
 unsafe fn init(argc: isize, argv: *const *const u8, sigpipe: u8) {
     unsafe {