[PATCH v1] diff: document -U without <n> as using default context

[Tian Yuchen <cat@malon.dev>]

The documentation for '-U<n>' implies that the numeric value '<n>' is
mandatory. However, the command line parser has historically accepted
'-U' without a number.

Strictly requiring a number for '-U' would break existing tests
(e.g., in 't4013') and likely disrupt user scripts relying on this
undocumented behavior.

Since we are retaining this fallback behavior for backward compatibility,
update the documentation to explicitly state that '<n>' can be omitted
for the short option '-U'.

Signed-off-by: Tian Yuchen <cat@malon.dev>
---
 Documentation/diff-context-options.adoc | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Documentation/diff-context-options.adoc b/Documentation/diff-context-options.adoc
index e161260358..655496ec3a 100644
--- a/Documentation/diff-context-options.adoc
+++ b/Documentation/diff-context-options.adoc
@@ -1,4 +1,4 @@
-`-U<n>`::
+`-U[<n>]`::
 `--unified=<n>`::
 	Generate diffs with _<n>_ lines of context. Defaults to `diff.context`
 	or 3 if the config option is unset.
-- 
2.43.0