Document namespace is a combination of name and assembly of the schema, separate by a comma (and a space).

Step-1. To find this you need to go to BizTalk Admin console then navigate to the application where you have deployed your schema DLL. Click on the Schemas (in the tree structure on the left) to list all schemas. Sort them as needed to find your schema. (It has to be deployed for this to work.)

Step-2. Double click on the schema for which you want the Document Spec Name; you will find “General” tab on the left hand side of the window.

Step-3. On this General tab, “Name”, “Assembly” and “Type” would be listed, read-only, but you can copy-paste from them.

Step 4 – Open NotePad++ or your favorite editor,
a) copy the value from “Name”,
b) type a comma followed by a space
c) copy the value from “Assembly”

Example from above:

Name: DemoSchema.Sample1
Assembly: DemoSchema, Version=1.0.0.0, Culture=neutral, PublicKeyToken=984c0907a0e7fc84

Thus, the created document spec name would be:
DemoSchema.Sample1, DemoSchema, Version=1.0.0.0, Culture=neutral, PublicKeyToken=984c0907a0e7fc84

Above steps taken from https://stackoverflow.com/questions/12319987/documentspecnames and elaborated a little.

NOTE: You can also get the Target Namespace from this screen.
If you want to filter on a message by MessageType, use the pattern namespace#rootElement.