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>