Merge branch 'mg/diff-uiconfig-doc' into maint

* mg/diff-uiconfig-doc:
  config.txt,diff-options.txt: porcelain vs. plumbing for color.diff

Documentation/config.txt

@@ -706,9 +706,16 @@ 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
@@ -794,11 +801,15 @@ 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

@@ -124,12 +124,19 @@ 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.