docs: add example for a couple of from_slice functions

Mostly to make it clear the examples like the following are not valid:

    RelativeDname::from_slice(b"_submissions._tcp");

src/base/name/dname.rs

@@ -164,6 +164,16 @@ impl Dname<[u8]> {
 
     /// Creates a domain name from an octets slice.
     ///
+    /// Note that the input must be in wire format, as shown below.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// Dname::from_slice(b"\x07example\x03com");
+    /// ```
+    ///
+    /// # Errors
+    ///
     /// This will only succeed if `slice` contains a properly encoded
     /// absolute domain name.
     pub fn from_slice(slice: &[u8]) -> Result<&Self, DnameError> {

src/base/name/relative.rs

@@ -136,6 +136,14 @@ impl RelativeDname<[u8]> {
     }
 
     /// Creates a relative domain name from an octet slice.
+    ///
+    /// Note that the input must be in wire format, as shown below.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// RelativeDname::from_slice(b"\x0c_submissions\x04_tcp");
+    /// ```
     pub fn from_slice(slice: &[u8]) -> Result<&Self, RelativeDnameError> {
         Self::check_slice(slice)?;
         Ok(unsafe { Self::from_slice_unchecked(slice) })