Documentation: declare "core.ignoreCase" as internal variable

Documentation: declare "core.ignoreCase" as internal variable

The current description of "core.ignoreCase" reads like an option which
is intended to be changed by the user while it's actually expected to
be set by Git on initialization only. Subsequently, Git relies on the
proper configuration of this variable, as noted by Bryan Turner [1]:

    Git on a case-insensitive filesystem (APFS, HFS+, FAT32, exFAT,
    vFAT, NTFS, etc.) is not designed to be run with anything other
    than core.ignoreCase=true.

[1] https://marc.info/?l=git&m=152998665813997&w=2
    mid:CAGyf7-GeE8jRGPkME9rHKPtHEQ6P1+ebpMMWAtMh01uO3bfy8w@mail.gmail.com

Signed-off-by: Marc Strapetz <marc.strapetz@syntevo.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>