docs: Distinguish replace layering action from document replacement
docs: Distinguish replace layering action from document replacement

Add a blurb to the replacement documentation that differentiates
the layering replace action from document replacement to avoid
conflation of the terms.

Change-Id: Ie3da4645c5fa4ed0eab95184443acbb01c5b2740

att-comdev/deckhand,att-comdev/deckhand