Finding Matching DLLs
There is no documentation, and unfortunately it's sometimes an educated guess which DLL contains the code you need to reference.
Since you know the Class/Namespace you are looking for, you can use a search tool to search through all the DLLs on the filesystem to find a match. I have found this to be the quickest way to narrow down to at least a set of matching DLLs. My personal preference is GrepWin from Stefan Tools, use whatever similar tools you prefer:
Note that this is a grep tool and you can use regular expressions if you need to find matches using some extra logic.
Verifying using a Decompiler
You can use a decompiler to confirm the match is correct, or if you find multiple matches load them all to find the correct DLL. My personal preference is JetBrains dotPeek Decompiler but use which ever similar tool you prefer. You can use the
Go to Everything / Type (Ctrl + T) or
Go to String (Ctrl + Alt + T) from the navigate menu to help find the type.
If you are already inspecting code in a decompiler then it's possible to sometimes figure out the DLL name from the
using statements that are referenced in the code. It may also be possible to inspect the references:
Otherwise over time with experience, you learn which DLLs (most likely) contain the code you need. I've found the above techniques to serve me well though.