Address cargo doc warnings.

This commit fixes warnings emitted by `cargo doc`.

Tested via `cargo doc --open --document-private-items` and manually
inspecting the affected output (i.e. `trait.EncodableTextChunk.html`,
`transform/palette/index.html`, and `enum.U32ValueKind.html`).

src/decoder/transform/palette.rs

@@ -5,7 +5,7 @@
 //!
 //! To achieve higher throughput, `create_rgba_palette` combines entries from
 //! `PLTE` and `trNS` chunks into a single lookup table.  This is based on the
-//! ideas explored in https://crbug.com/706134.
+//! ideas explored in <https://crbug.com/706134>.
 //!
 //! Memoization is a trade-off:
 //! * On one hand, memoization requires spending X ns before starting to call

src/text_metadata.rs

@@ -145,7 +145,7 @@ pub(crate) enum TextDecodingError {
 
 /// A generalized text chunk trait
 pub trait EncodableTextChunk {
-    /// Encode text chunk as Vec<u8> to a `Write`
+    /// Encode text chunk as `Vec<u8>` to a `Write`
     fn encode<W: Write>(&self, w: &mut W) -> Result<(), EncodingError>;
 }