docs: Add information about configuring delta pager

docs/config.md

@@ -386,6 +386,19 @@ command will be executed directly. For example:
 ui.pager = ["sh", "-c", "diff-so-fancy | less -RFX"]
 ```
 
+Some formatters (like [`delta`](https://github.com/dandavison/delta)) require
+git style diffs for formatting. You can select this option on each invocation
+of `jj diff` by using the `--git` flag. You can also configure this style of
+diff as the default with the `ui.diff` setting. For example:
+
+```toml
+[ui]
+pager = "delta"
+
+[ui.diff]
+format = "git"
+```
+
 ## Aliases
 
 You can define aliases for commands, including their arguments. For example: