docs: add customization example of color-words diff style

The default style is compatible with various terminal emulators, but I suspect
that many people wouldn't like underlines. Let's add an example to override it.

CHANGELOG.md

@@ -63,6 +63,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 * New diff option `jj diff --name-only` allows for easier shell scripting.
 
+* In color-words diffs, hunks are now highlighted with underline. See [diff
+  colors and styles](docs/config.md#diff-colors-and-styles) for customization.
+
 * `jj git push -c <arg>` can now accept revsets that resolve to multiple
   revisions. This means that `jj git push -c xyz -c abc` is now equivalent to
   `jj git push -c 'all:(xyz | abc)'`.

docs/config.md

@@ -166,6 +166,18 @@ useful reminder to fill in things like BUG=, TESTED= etc.
 ui.default-description = "\n\nTESTED=TODO"
 ```
 
+### Diff colors and styles
+
+In color-words diffs, hunks are rendered with underline. You can override the
+default style with the following keys:
+
+```toml
+[colors]
+# Highlight hunks with background
+"diff removed token" = { bg = "#221111", underline = false }
+"diff added token" = { bg = "#002200", underline = false }
+```
+
 ### Diff format
 
 ```toml