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>

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