Documentation/diff-format: state in which cases porcelain status is T

Documentation/diff-format: state in which cases porcelain status is T

Porcelain status letter T is documented as "type of the file", which
is technically correct but not enough information for users that are
not so familiar with this term from systems programming. In particular,
given that the only supported file types are "regular file", "symbolic
link" and "submodule", the term "file type" is surely opaque to the
many(?) users who are not aware that symbolic links can be tracked -
I thought that a "chmod +x" could cause the T status (wrong, it's M).

Explicitly document the three file types so users know if/how they
want to handle this.

Signed-off-by: Johannes Altmanninger <aclopte@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>