config.txt: document behavior of backslashes in subsections

Unrecognized escape sequences are invalid in values:

  $ git config -f - --list <<EOF
  [foo]
    bar = "\t\\\y\"\u"
  EOF
  fatal: bad config line 2 in standard input

But in subsection names, the backslash is simply dropped if the
following character does not produce a recognized escape sequence:

  $ git config -f - --list <<EOF
  [foo "\t\\\y\"\u"]
    bar = baz
  EOF
  foo.t\y"u.bar=baz

Although it would be nice for subsection names and values to have
consistent behavior, changing the behavior for subsection names is a
nonstarter since it would cause existing, valid config files to
suddenly be interpreted differently.

Signed-off-by: Dave Borowitz <dborowitz@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -41,11 +41,13 @@ in the section header, like in the example below:
 --------
 
 Subsection names are case sensitive and can contain any characters except
-newline (doublequote `"` and backslash can be included by escaping them
-as `\"` and `\\`, respectively).  Section headers 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.
+newline and the null byte. Doublequote `"` and backslash can be included
+by escaping them as `\"` and `\\`, respectively. Backslashes preceding
+other characters are dropped when reading; for example, `\t` is read as
+`t` and `\0` is read as `0` Section headers 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 a deprecated `[section.subsection]` syntax. With this
 syntax, the subsection name is converted to lower-case and is also