git-config: document accidental multi-line setting in deprecated syntax

git-config: document accidental multi-line setting in deprecated syntax

The bug was noticed when writing the previous patch; a fix for this bug
is not easy though: If we choose to ignore the case of the subsection
(and revert most of the code of the previous patch, just keeping
s/strncasecmp/strcmp/), then we'd introduce new sections using the
new syntax, such that

 --------
   [section.subsection]
     key = value1
 --------

  git config section.Subsection.key value2

would result in

 --------
   [section.subsection]
     key = value1
   [section.Subsection]
     key = value2
 --------

which is even more confusing. A proper fix would replace the first
occurrence of 'key'. As the syntax is deprecated, let's prefer to not
spend time on fixing the behavior and just document it instead.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>