doc: clarify that --word-diff operates on line-level hunks

The --word-diff documentation describes the output modes and
word-regex mechanics but does not explain that word-diff operates
within the hunks produced by the line-level diff rather than
performing an independent word-stream comparison.  This can
surprise users when the line-level alignment causes word-level
changes to appear even though the words in both files are
identical.

Add an implementation note explaining the two-stage relationship
and that the output may change if Git acquires a different
implementation in the future.

Signed-off-by: Michael Montalbo <mmontalbo@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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