doc: gitignore: clarify pattern base for info/exclude and core.excludesFile

doc: gitignore: clarify pattern base for info/exclude and core.excludesFile

The pattern format section describes how patterns are interpreted
relative to the location of a .gitignore file, but does not mention
the behavior for exclude sources outside the working tree.

Clarify that patterns from $GIT_DIR/info/exclude and core.excludesFile
are treated as if they are specified at the root of the working tree,
so a leading '/' anchors matches at the repository root.

Reported-by: Dan Drake <dan@dandrake.org>
Signed-off-by: Shreyansh Paliwal <shreyanshpaliwalcmsmn@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>