Changed docs for `core.ignorecase` to mention HFS

Changed docs for git-config's `core.ignorecase` entry to mention HFS in addition to FAT.  Rationale is that the primary user base of Git nowadays is running Linux, OS X or Windows (> XP)— and the typical filesystems you'd find there are respectively ext3/4, HFS+, and NTFS.  Of those, only HFS+ is case-insensitive (by default).  Left FAT there since it's still used for some things, like flash drives.

Also generalized the statement “that are not case sensitive” to “that are typically not case sensitive” because HFS+ drives are capable of being formatted in either case-insensitive or case-sensitive mode, but OS X defaults to case-insensitive and (from my personal experience) case-sensitive HFS+ is rarely used— it tends to cause crashes and other issues with a good number of common OS X apps, which weren't written for it.