diff: document -U without <n> as using default context

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.

Hence we retain this fallback behavior for backward compatibility, but
document it as such.

Signed-off-by: Tian Yuchen <cat@malon.dev>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: Tian Yuchen

Documentation/diff-context-options.adoc

@@ -1,7 +1,9 @@
 `-U<n>`::
 `--unified=<n>`::
-	Generate diffs with _<n>_ lines of context. Defaults to `diff.context`
-	or 3 if the config option is unset.
+	Generate diffs with _<n>_ lines of context. The number of context
+	lines defaults to `diff.context` or 3 if the configuration variable
+	is unset. (`-U` without `<n>` is silently accepted as a synonym for
+	`-p` due to a historical accident).
 
 `--inter-hunk-context=<n>`::
 	Show the context between diff hunks, up to the specified _<number>_

Documentation/diff-options.adoc

@@ -127,8 +127,10 @@ endif::git-log[]
 
 `-U<n>`::
 `--unified=<n>`::
-	Generate diffs with _<n>_ lines of context instead of
-	the usual three.
+	Generate diffs with _<n>_ lines of context. The number of context
+	lines defaults to `diff.context` or 3 if the configuration variable
+	is unset. (`-U` without `<n>` is silently accepted as a synonym for
+	`-p` due to a historical accident).
 ifndef::git-format-patch[]
 	Implies `--patch`.
 endif::git-format-patch[]