In my opinion, assembly instructions may or may not be "work instructions."
"Work instructions" to me implies being part of an entire "process" or "system" with the concommitant checks and balances. It means there is an element of training and evaluation on following the instructions.
"Assembly Instructions" imply something done on an ad hoc basis (bad English instructions on assembling toys or furniture on Christmas Eve) with no consideration for whether the proper tools and training are supplied to do the job correctly.
Further, "assembly instructions" imply only the process of putting finished components together. Whereas "work instruction" encompasses all processes from digging ore out of the ground to packaging a finished product for sale to a retail customer.
Bottom line:
Assembly instructions may be a special type of work instruction if the workplace, tools, and training are supplied by management.