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>

Documentation/config.txt

@@ -14,14 +14,72 @@ dot-separated segment and the section name is everything before the last
 dot. The variable names are case-insensitive and only alphanumeric
 characters are allowed. Some variables may appear multiple times.
 
+Syntax
+~~~~~~
+
 The syntax is fairly flexible and permissive; whitespaces are mostly
-ignored. The '#' and ';' characters begin comments to the end of line,
-blank lines are ignored, lines containing strings enclosed in square
-brackets start sections and all the other lines 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". String values may be entirely or partially
-enclosed in double quotes; some variables may require special value format.
+ignored.  The '#' and ';' characters begin comments to the end of line,
+blank lines are ignored.
+
+The file consists of sections and variables.  A section begins with
+the name of the section in square brackets and continues until the next
+section begins.  Section names are not case sensitive.  Only alphanumeric
+characters, '`-`' and '`.`' are allowed in section names.  Each variable
+must belong to some section, which means that there must be section
+header before first setting of a variable.
+
+Sections can be further divided into subsections.  To begin a subsection
+put its name in double quotes, separated by space from the section name,
+in the section header, like in example below:
+
+--------
+	[section "subsection"]
+
+--------
+
+Subsection names can contain any characters (doublequote '`"`', backslash
+'`\`' and newline have to be entered escaped as '`\"`', '`\\`' and '`\n`',
+respecitvely) and are case sensitive.  Section header cannot span multiple
+lines.  Variables may belong directly to a section or to a given subsection.
+You can have `[section]` if you have `[section "subsection"]`, but you
+don't need to.
+
+There is also (case insensitive) alternative `[section.subsection]` syntax.
+In this syntax subsection names follow the same restrictions as for section
+name.
+
+All the other lines 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 and only alphanumeric
+characters and '`-`' are allowed.  There can be more than one value
+for a given variable; we say then that variable is multivalued.
+
+Leading and trailing whitespace in a variable value is discarded.
+Internal whitespace within a variable value is retained verbatim.
+
+The values following the equals sign in variable assign are all either
+a string, an integer, or a boolean.  Boolean values may be given as yes/no,
+0/1 or true/false.  Case is not significant in boolean values, when
+converting value to the canonical form using '--bool' type specifier;
+`git-repo-config` will ensure that the output is "true" or "false".
+
+String values may be entirely or partially enclosed in double quotes.
+You need to enclose variable value in double quotes if you want to
+preserve leading or trailing whitespace, or if variable value contains
+beginning of comment characters (if it contains '#' or ';').
+Double quote '`"`' and backslash '`\`' characters in variable value must
+be escaped: use '`\"`' for '`"`' and '`\\`' for '`\`'.
+
+The following escape sequences (beside '`\"`' and '`\\`') are recognized:
+'`\n`' for newline character (NL), '`\t`' for horizontal tabulation (HT, TAB)
+and '`\b`' for backspace (BS).  No other char escape sequence, nor octal
+char sequences are valid.
+
+Variable value ending in a '`\`' is continued on the next line in the
+customary UNIX fashion.
+
+Some variables may require special value format.
 
 Example
 ~~~~~~~
@@ -40,6 +98,10 @@ Example
 		remote = origin
 		merge = refs/heads/devel
 
+	# Proxy settings
+	[core]
+		gitProxy="ssh" for "ssh://kernel.org/"
+		gitProxy=default-proxy ; for the rest
 
 Variables
 ~~~~~~~~~