documentation: clarify direction of core.autocrlf

documentation: clarify direction of core.autocrlf

The description for core.autocrlf refers to reads from / writes to
"the filesystem", the only use of this rather ambiguous term, which
technically could be referring to the git object database. (All other
mentions are part of phrases such as "..filesystems (like NFS)..").

Other sections, including the section on core.safecrlf, use the term
"work tree" for the same purpose as the term "the filesystem" is used in
the core.autocrlf section, so that seems like a good alternative, which
makes it clearer what direction the addition/removal of CR characters
occurs in.

Signed-off-by: Will Palmer <wmpalmer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>