Add some documentation

src/content.rs

@@ -27,7 +27,15 @@ impl Content {
         Operation::start(&mut self.buf, operator)
     }
 
-    /// Return the raw constructed byte stream.
+    /// Return the buffer of the content stream.
+    ///
+    /// The buffer is essentially a thin wrapper around two objects:
+    /// - A [`Limits`] object, which can optionally be used to keep
+    ///   track of data such as the largest used integer or
+    ///   the longest string used in the content streams, which is useful information
+    ///   for some export modes.
+    /// - The actual underlying data of the content stream, which can be written
+    ///   to a chunk (and optionally apply a filter before doing so).
     pub fn finish(mut self) -> Buf {
         if self.buf.last() == Some(&b'\n') {
             self.buf.inner.pop();