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>

Documentation/diff-options.adoc

@@ -455,6 +455,14 @@ 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.
++
+The `--word-diff` option operates by taking the same line-by-line
+diff that is produced without the option and computing
+word-by-word changes within each hunk.  This may produce a
+larger diff than a dedicated word-diff tool would.  If Git
+acquires a different implementation in the future, the output
+may change.  Note that this is similar to the `--diff-algorithm`
+option, which may also change the output.
 
 `--word-diff-regex=<regex>`::
 	Use _<regex>_ to decide what a word is, instead of considering