Standardise and add safety comments.

These will soon be required by a lint.

Bug: 290018030
Test: m rust
Change-Id: If62281a5eb2160061a11f28588b7a14e32ce265b
diff --git a/microdroid_manager/src/main.rs b/microdroid_manager/src/main.rs
index 1cdcde1..9548936 100644
--- a/microdroid_manager/src/main.rs
+++ b/microdroid_manager/src/main.rs
@@ -193,7 +193,7 @@
 
 /// Prepares a socket file descriptor for the vm payload service.
 ///
-/// # Safety requirement
+/// # Safety
 ///
 /// The caller must ensure that this function is the only place that claims ownership
 /// of the file descriptor and it is called only once.
@@ -267,6 +267,8 @@
     if Path::new(ENCRYPTEDSTORE_BACKING_DEVICE).exists() {
         let mountpoint = CString::new(ENCRYPTEDSTORE_MOUNTPOINT).unwrap();
 
+        // SAFETY: `mountpoint` is a valid C string. `syncfs` and `close` are safe for any parameter
+        // values.
         let ret = unsafe {
             let dirfd = libc::open(
                 mountpoint.as_ptr(),