Documentation/config.txt: explain multi-valued variables once

The syntax section repeats what the preamble explained already.
That a variable can have multiple values is more about what a
variable is than the syntax of the file.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -14,7 +14,8 @@ the fully qualified variable name of the variable itself is the last
 dot-separated segment and the section name is everything before the last
 dot. The variable names are case-insensitive, allow only alphanumeric
 characters and `-`, and must start with an alphabetic character.  Some
-variables may appear multiple times.
+variables may appear multiple times; we say then that the variable is
+multivalued.
 
 Syntax
 ~~~~~~
@@ -56,9 +57,7 @@ header) are recognized as setting variables, in the form
 'name = value'.  If there is no equal sign on the line, the entire line
 is taken as 'name' and the variable is recognized as boolean "true".
 The variable names are case-insensitive, allow only alphanumeric characters
-and `-`, and must start with an alphabetic character.  There can be more
-than one value for a given variable; we say then that the variable is
-multivalued.
+and `-`, and must start with an alphabetic character.
 
 Leading and trailing whitespace in a variable value is discarded.
 Internal whitespace within a variable value is retained verbatim.