config.txt,diff-options.txt: porcelain vs. plumbing for color.diff

Reading the diff-family and config man pages one may think that the
color.diff and color.ui settings apply to all diff commands. Make it
clearer that they do not apply to the plumbing variants
diff-{files,index,tree}.

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -708,9 +708,16 @@ second is the background.  The position of the attribute, if any,
 doesn't matter.
 
 color.diff::
-	When set to `always`, always use colors in patch.
-	When false (or `never`), never.  When set to `true` or `auto`, use
-	colors only when the output is to the terminal. Defaults to false.
+	Whether to use ANSI escape sequences to add color to patches.
+	If this is set to `always`, linkgit:git-diff[1],
+	linkgit:git-log[1], and linkgit:git-show[1] will use color
+	for all patches.  If it is set to `true` or `auto`, those
+	commands will only use color when output is to the terminal.
+	Defaults to false.
++
+This does not affect linkgit:git-format-patch[1] nor the
+'git-diff-{asterisk}' plumbing commands.  Can be overridden on the
+command line with the `--color[=<when>]` option.
 
 color.diff.<slot>::
 	Use customized color for diff colorization.  `<slot>` specifies
@@ -796,11 +803,15 @@ color.status.<slot>::
 	color.branch.<slot>.
 
 color.ui::
-	When set to `always`, always use colors in all git commands which
-	are capable of colored output. When false (or `never`), never. When
-	set to `true` or `auto`, use colors only when the output is to the
-	terminal. When more specific variables of color.* are set, they always
-	take precedence over this setting. Defaults to false.
+	This variable determines the default value for variables such
+	as `color.diff` and `color.grep` that control the use of color
+	per command family. Its scope will expand as more commands learn
+	configuration to set a default for the `--color` option.  Set it
+	to `always` if you want all output not intended for machine
+	consumption to use color, to `true` or `auto` if you want such
+	output to use color when written to the terminal, or to `false` or
+	`never` if you prefer git commands not to use color unless enabled
+	explicitly with some other configuration or the `--color` option.
 
 commit.status::
 	A boolean to enable/disable inclusion of status information in the

Documentation/diff-options.txt

@@ -120,12 +120,19 @@ any of those replacements occurred.
 
 --color[=<when>]::
 	Show colored diff.
-	The value must be always (the default), never, or auto.
+	The value must be `always` (the default for `<when>`), `never`, or `auto`.
+	The default value is `never`.
+ifdef::git-diff[]
+	It can be changed by the `color.ui` and `color.diff`
+	configuration settings.
+endif::git-diff[]
 
 --no-color::
-	Turn off colored diff, even when the configuration file
-	gives the default to color output.
-	Same as `--color=never`.
+	Turn off colored diff.
+ifdef::git-diff[]
+	This can be used to override configuration settings.
+endif::git-diff[]
+	It is the same as `--color=never`.
 
 --word-diff[=<mode>]::
 	Show a word diff, using the <mode> to delimit changed words.