fast-import: improve documentation for path quoting

fast-import: improve documentation for path quoting

It describes what characters cannot be in an unquoted path, but not
their semantics. Reframe it as a definition of unquoted paths. From the
perspective of the parser, whether it starts with `"` is what defines
whether it will parse it as quoted or unquoted.

The restrictions on characters in unquoted paths (with starting-", LF,
and spaces) are explained in the quoted paragraph. Move it to the
unquoted paragraph and reword.

The restriction that the source paths of filecopy and filerename cannot
contain SP is only stated in their respective sections. Restate it in
the <path> section.

Signed-off-by: Thalia Archibald <thalia@archibald.dev>
Signed-off-by: Junio C Hamano <gitster@pobox.com>