doc: note that default signal handling resets the signal mask

`InitializeOncePerProcess()` without `kNoDefaultSignalHandling` calls
`pthread_sigmask(SIG_SETMASK, ...)` with a set containing only SIGUSR1,
which unblocks every signal the embedder had blocked on the calling
thread. Say so in the flag's documentation.

Refs: #44121
Signed-off-by: Shelley Vohr <shelley.vohr@gmail.com>
PR-URL: #65877
Refs: #25777
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>

Author: codebytere

src/node.h

@@ -239,8 +239,10 @@ enum Flags : uint32_t {
   kNoICU = 1 << 3,
   // Do not modify stdio file descriptor or TTY state.
   kNoStdioInitialization = 1 << 4,
-  // Do not register Node.js-specific signal handlers
-  // and reset other signal handlers to default state.
+  // Do not register Node.js-specific signal handlers, reset other signal
+  // handlers to default state, or replace the calling thread's signal mask
+  // (without this flag, POSIX builds with the inspector set it to block
+  // SIGUSR1 and nothing else).
   kNoDefaultSignalHandling = 1 << 5,
   // Do not perform V8 initialization.
   kNoInitializeV8 = 1 << 6,