Add a note about `OsRng` blocking in early init.
diff --git a/src/os.rs b/src/os.rs
index e9ab758..c0010fb 100644
--- a/src/os.rs
+++ b/src/os.rs
@@ -26,7 +26,9 @@
/// - iOS: calls SecRandomCopyBytes as /dev/(u)random is sandboxed.
/// - PNaCl: calls into the `nacl-irt-random-0.1` IRT interface.
///
-/// This does not block.
+/// This usually does not block. On some systems (e.g. FreeBSD, OpenBSD,
+/// Max OS X, and modern Linux) this may block very early in the init
+/// process, if the CSPRNG has not seeded yet.
pub struct OsRng(imp::OsRng);
impl OsRng {
