Documentation/config.txt: describe the structure first and then meaning

Documentation/config.txt: describe the structure first and then meaning

A line can be continued via a backquote-LF and can be chomped at a
comment character.  But that is not specific to string-typed values.
It is common to all, just like unquoted leading and trailing
whitespaces are stripped and inter-word spacing are retained.

Move the description around and desribe these structural rules
first, then introduce the double-quote facility as a way to override
them, and finally mention various types of values.

Note that these structural rules only apply to the value part of the
configuration file.  E.g.

    [aSection] \
        name \
	= value

does not work, because the rules kick in only after seeing "name =".
Both the original and the updated text are phrased in an awkward way
by singling out the "value" part of the line because of this.

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