Documentation/config.txt: Document config file syntax better

Documentation/config.txt: Document config file syntax better

Separate part of Documentation/config.txt which deals with git config file
syntax into "Syntax" subsection, and expand it.  Add information about
subsections, boolean values, escaping and escape sequences in string
values, and continuing variable value on the next line.

Add also proxy settings to config file example to show example of
partially enclosed in double quotes string value.

Parts based on comments by Junio C Hamano, Johannes Schindelin,
config.c, and the smb.conf(5) man page.

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>