DOCS-14824: Clarify random part of objectId

Author: npentrel

source/includes/fact-ObjectId-construct.rst

@@ -1,7 +1,8 @@
 - a 4-byte *timestamp value*, representing the ObjectId's creation,
   measured in seconds since the Unix epoch
 
-- a 5-byte *random value*
+- a 5-byte *random value* generated once per process. This random value
+  is unique to the machine and process.
 
 - a 3-byte *incrementing counter*, initialized to a random value