Add note to item-pipeline documentation explaining order

[bjlange]

As a new Scrapy user I had trouble hunting down what the numbers meant in the ITEM_PIPELINES setting. I added a note to the item-pipeline documentation to make it more clear.

[kmike]

Hi @bjlange,

Good catch. But it is still unclear if these numbers mean "order" (pipelines with large numbers are applied last) or "priority" (pipelines with large numbers are applied first)

[bjlange]

Good call. I made a revision.

For anyone curious who finds this PR in the future, the source code that makes it work this way is located here as of posting.

[dangra]

LGTM

[kmike]

Thanks!