docs(fspy): scope macOS resize safety

Co-authored-by: GPT-5 Codex <codex@openai.com>

Author: wan9chi

crates/fspy_shm/src/macos/mod.rs

@@ -74,10 +74,9 @@ pub fn create(size: usize) -> io::Result<Shm> {
 pub fn open(id: &str) -> io::Result<Shm> {
     // `rustix::shm::open` sets `FD_CLOEXEC` on the returned descriptor.
     let fd = shm::open(id, OFlags::RDWR, Mode::empty()).map_err(io::Error::from)?;
-    // Another process that opens this object for writing can resize it. If it
-    // shrinks the object after `fstat`, `mmap` rejects the requested size and
-    // `open` returns an error. Fspy sets the size before sharing `id` and never
-    // changes it afterward.
+    // If another process shrinks the object before `mmap`, `mmap` returns an
+    // error. If it resizes the object after `mmap`, `open` does not access the
+    // mapped pages. A concurrent resize cannot make `open` access invalid memory.
     let stat = fstat(&fd)?;
     let size = usize::try_from(stat.st_size)
         .map_err(|_| io::Error::new(io::ErrorKind::InvalidData, "invalid shared-memory size"))?;