doc: Add comment to helper functions marked as pub

Add comment on helper function `new` of `MmapRegion` and `raw_os_error`
of `Error`.

Signed-off-by: Ruoqing He <[email protected]>

Author: RuoqingHe

src/mmap_unix.rs

@@ -455,9 +455,10 @@ mod tests {
 
     type MmapRegion = super::MmapRegion<()>;
 
-    // Adding a helper method to extract the errno within an Error::Mmap(e), or return a
-    // distinctive value when the error is represented by another variant.
     impl Error {
+        /// Helper method to extract the errno within an
+        /// `Error::Mmap(e)`. Returns `i32::MIN` if `self` is any
+        /// other variant.
         pub fn raw_os_error(&self) -> i32 {
             match self {
                 Error::Mmap(e) => e.raw_os_error().unwrap(),

src/mmap_xen.rs

@@ -1063,6 +1063,7 @@ mod tests {
     }
 
     impl MmapRegion {
+        /// Create an `MmapRegion` with specified `size` at GuestAdress(0)
         pub fn new(size: usize) -> Result<Self> {
             let range = MmapRange::new_unix(size, None, GuestAddress(0));
             Self::from_range(range)