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 <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: mstrap

Documentation/config.txt

@@ -390,16 +390,19 @@ core.hideDotFiles::
 	default mode is 'dotGitOnly'.
 
 core.ignoreCase::
-	If true, this option enables various workarounds to enable
+	Internal variable which enables various workarounds to enable
 	Git to work better on filesystems that are not case sensitive,
-	like FAT. For example, if a directory listing finds
-	"makefile" when Git expects "Makefile", Git will assume
+	like APFS, HFS+, FAT, NTFS, etc. For example, if a directory listing
+	finds "makefile" when Git expects "Makefile", Git will assume
 	it is really the same file, and continue to remember it as
 	"Makefile".
 +
 The default is false, except linkgit:git-clone[1] or linkgit:git-init[1]
 will probe and set core.ignoreCase true if appropriate when the repository
 is created.
++
+Git relies on the proper configuration of this variable for your operating
+and file system. Modifying this value may result in unexpected behavior.
 
 core.precomposeUnicode::
 	This option is only used by Mac OS implementation of Git.