column doc: rewrite documentation for column.ui

The configuration option column.ui is very poorly documented, and it is
unclear what the defaults are, and what option can be combined with
what.  Rewrite it by splitting up the options into three sections
clearly showing how COL_ENABLED, COL_LAYOUT_MASK, and COL_DENSE work.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -930,6 +930,9 @@ column.ui::
 	This variable consists of a list of tokens separated by spaces
 	or commas:
 +
+These options control when the feature should be enabled
+(defaults to 'never'):
++
 --
 `always`;;
 	always show in columns
@@ -937,19 +940,30 @@ column.ui::
 	never show in columns
 `auto`;;
 	show in columns if the output is to the terminal
+--
++
+These options control layout (defaults to 'column').  Setting any
+of these implies 'always' if none of 'always', 'never', or 'auto' are
+specified.
++
+--
 `column`;;
-	fill columns before rows (default)
+	fill columns before rows
 `row`;;
 	fill rows before columns
 `plain`;;
 	show in one column
+--
++
+Finally, these options can be combined with a layout option (defaults
+to 'nodense'):
++
+--
 `dense`;;
 	make unequal size columns to utilize more space
 `nodense`;;
 	make equal size columns
 --
-+
-This option defaults to 'never'.
 
 column.branch::
 	Specify whether to output branch listing in `git branch` in columns.