Assembly drawings are the master prints to completely assemble a unit.
They should contain (at a minimum):
The quality standard to be used for assembly and inspection (usually IPC-A-610 class 2; class 3 is difficult to achieve unless you are willing to pay a premium).
A reference to the schematic with current revision (which you should also supply).
A reference to the Bill of Materials with current revision which, once more, you should supply. The Bill of Materials should specify the bare board and revision information.
Any special instructions (not everything required to completely assemble the board will be in the normal tool outputs):
Bonding of components perhaps for high vibration environments where you should also specify the material type (at least the standard to which they must be made). I usually specify a particular bonding agent. You should clearly identify on this drawing which components are to have instructions applied to (using a callout)
Any modifications to be performed - we do not spin a PCB for every modification which may require a wire or two.
Any conformal coat requirements, including masking information (you can do this on a fabrication layer, but that layer information needs to be called out).
Any test requirements (you may need to provide test equipment or work with the assembler to define the tests).
Any other information: perhaps some Kapton (r) tape needs to be applied to avoid short circuits.
A view of any surface with components with reference designators clearly visible.
An isometric view is useful for some information.
The applicable standard is IPC-D-325.
I usually require a certificate of conformance to the instructions provided and that assembly has been carried out to the quality standard referenced.
Note that I also generate fabrication master prints in addition to gerbers / ODB++ and the other various outputs; that is a subject in its own right.
There are other things that may end up on the drawing, but this should get you going; I do not use Eagle, so I do not know if there is a simple way of doing this (I use Altium with the drafstman tool)
Edited for comments.
Most passives are commodity parts; it is your responsibility to identify the parts that can be eaaily changed.