config.txt: document behavior of backslashes in subsections

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>