docs: correctly mention compression level

- mention the difference in compression level range
when using `zlib` vs `miniz` backends

src/lib.rs

@@ -192,6 +192,10 @@ impl Compression {
     ///
     /// The integer here is typically on a scale of 0-9 where 0 means "no
     /// compression" and 9 means "take as long as you'd like".
+    /// 
+    /// It is worth noting that `flate2` supports both `zlib` and `miniz` backends for compression,
+    /// where `miniz` is enabled by default, and `zlib` can be enabled by the `zlib` feature.
+    /// The `zlib` backend supports levels 0-9, while the `miniz` backend supports levels 0-10.
     pub const fn new(level: u32) -> Compression {
         Compression(level)
     }
@@ -208,12 +212,16 @@ impl Compression {
     }
 
     /// Optimize for the size of data being encoded.
+    /// 
+    /// The maximum compression is always returned as 9 for consistency
+    /// with the `zlib` backend, but the `miniz` backend supports a 
+    /// maximum compression level of 10, and is enabled by default.
     pub const fn best() -> Compression {
         Compression(9)
     }
 
     /// Returns an integer representing the compression level, typically on a
-    /// scale of 0-9
+    /// scale of 0-9. With `miniz` backend, this can be 0-10.
     pub fn level(&self) -> u32 {
         self.0
     }