Which schema is better?
You should definitely two separate tables!


What if, in the future, documents get linked to something else? 


As it is anyway with your documents_relationship table, you are most likely going to have fields that apply to clients and not assets and vice versa! 

This will mean NULLs. Good frule of thumb IMHO, a field should never be NULL by design for its entire life. Note: this does not mean that NULLs should never be used!