devices: doc for the pseudo boot device

Doc for the pseudo boot device

Signed-off-by: Diana Popa <dpopa@amazon.com>

Author: dianpopa

src/vmm/src/devices/pseudo/boot_timer.rs

@@ -36,6 +36,7 @@ impl BootTimer {
 }
 
 impl BootTimer {
+    /// Create a device at a certain point in time.
     pub fn new(start_ts: TimestampUs) -> BootTimer {
         BootTimer { start_ts }
     }

src/vmm/src/devices/pseudo/mod.rs

@@ -1,6 +1,7 @@
 // Copyright 2020 Amazon.com, Inc. or its affiliates. All Rights Reserved.
 // SPDX-License-Identifier: Apache-2.0
 
+//! Implements Firecracker specific devices (e.g. signal when boot is completed).
 mod boot_timer;
 
 pub use self::boot_timer::BootTimer;