[rust] update mergedown doc comment

- fix more warnings

rust/src/plane/methods.rs

@@ -1020,18 +1020,24 @@ impl NcPlane {
             "NcPlane.move_below(NcPlane)"
         ]
     }
-
-    /// Merges `source` down onto this NcPlane.
+    /// Merge the NcPlane `source` down onto the current NcPlane (`self`).
+    ///
+    /// This is most rigorously defined as "write to `self` the frame that would
+    /// be rendered were the entire stack made up only of the specified subregion
+    /// of `source` and, below it, the subregion of `self` having the specified
+    /// origin.
     ///
-    /// Merging is independent of the position of both NcPlanes on the z-axis.
+    /// Merging is independent of the position of both planes on the z-axis.
     ///
     /// It is an error to define a subregion of zero area, or that is not
-    /// entirely contained within `source`.
+    /// entirely contained within `source`. It is an error to define a target
+    /// origin such that the projected subregion is not entirely contained
+    /// within `self`.
     ///
-    /// It is an error to define a target origin such that the projected
-    /// subregion is not entirely contained within 'dst'.
+    /// Behavior is undefined if both planes are equivalent. `self` is modified,
+    /// but `source` remains unchanged.
     ///
-    /// Behavior is undefined if both NcPlanes are equivalent.
+    /// neither `source` nor `self` may have sprixels.
     ///
     /// *C style function: [ncplane_mergedown()][crate::ncplane_mergedown].*
     pub fn mergedown(

rust/src/resizecb.rs

@@ -13,11 +13,7 @@ pub type NcResizeCbUnsafe = unsafe extern "C" fn(*mut NcPlane) -> NcIntResult;
 
 /// Converts [NcResizeCbUnsafe] to [NcResizeCb].
 pub fn ncresizecb_to_rust(resizecb: Option<NcResizeCbUnsafe>) -> Option<NcResizeCb> {
-    if let Some(cb) = resizecb {
-        Some(unsafe { core::mem::transmute(cb) })
-    } else {
-        None
-    }
+    resizecb.map(|cb| unsafe { core::mem::transmute(cb) })
 }
 
 /// Converts [NcResizeCb] to [NcResizeCbUnsafe].
@@ -25,11 +21,7 @@ pub fn ncresizecb_to_rust(resizecb: Option<NcResizeCbUnsafe>) -> Option<NcResize
 // waiting for https://github.com/rust-lang/rust/issues/53605
 // to make this function const, and then NcPlaneOptions constructors.
 pub fn ncresizecb_to_c(resizecb: Option<NcResizeCb>) -> Option<NcResizeCbUnsafe> {
-    if let Some(cb) = resizecb {
-        Some(unsafe { core::mem::transmute(cb) })
-    } else {
-        None
-    }
+    resizecb.map(|cb| unsafe { core::mem::transmute(cb) })
 }
 
 /// Enables the [NcResizeCb] methods.