color-words: expand docs with precise semantics

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/diff-options.txt

@@ -91,12 +91,17 @@ endif::git-format-patch[]
 	Turn off colored diff, even when the configuration file
 	gives the default to color output.
 
---color-words[=regex]::
-	Show colored word diff, i.e. color words which have changed.
+--color-words[=<regex>]::
+	Show colored word diff, i.e., color words which have changed.
+	By default, words are separated by whitespace.
 +
-Optionally, you can pass a regular expression that tells Git what the
-words are that you are looking for; The default is to interpret any
-stretch of non-whitespace as a word.
+When a <regex> is specified, every non-overlapping match of the
+<regex> is considered a word.  Anything between these matches is
+considered whitespace and ignored(!) for the purposes of finding
+differences.  You may want to append `|[^[:space:]]` to your regular
+expression to make sure that it matches all non-whitespace characters.
+A match that contains a newline is silently truncated(!) at the
+newline.
 
 --no-renames::
 	Turn off rename detection, even when the configuration