Components with multiple possible values - how to designate on schematic?
Well first of all decoupling cap values do matter :).  But to answer your question you should manage that with your bom.   Let's say you have a 100 ohm resistor, give it an internal part number.  Now under that part number on the bom you can assign multiple manufacturers part numbers as acceptable alternates.

I have a system that manages this for me in orcad CIS/CIP but you can keep track of it in excel.

Specing multiple values for a component sounds like a bit of a mess in production if you ask me, and a risk to the design.  Maybe there's a combo that will cause an issue.  I wouldn't allow it in my shop.

If you want to be more free with purchasing you can specify that substitutions are allowed if parts meet all of the same requirements or 1% ok where 5% is specified.  I would specify that you must approve all substitutions though.  After all the success of the design is your responsibility :)

Also unless they own the design, factories don't like to be making design decisions like let me out a 10k in here where he has a 1k.