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 a short note explaining the two-stage relationship.

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

Author: mmontalbo

Documentation/diff-options.adoc

@@ -455,6 +455,11 @@ endif::git-diff[]
 +
 Note that despite the name of the first mode, color is used to
 highlight the changed parts in all modes if enabled.
++
+Word diff works by finding word-level changes within each hunk of
+the line-level diff.  The line-level alignment determines which
+changed lines are compared to each other, which can affect the
+word-level output.
 
 `--word-diff-regex=<regex>`::
 	Use _<regex>_ to decide what a word is, instead of considering