improve xapi version doc

Author: milt

doc/xapi_version.md

@@ -2,6 +2,13 @@
 
 # xAPI Version Support
 
-As of v0.0.34 LRSPipe supports xAPI versions 1.0.3 and 2.0.0 using the `--source-xapi-version` and `--target-xapi-version` arguments. Note that when transferring statements from a 2.0.0 LRS to a 1.0.3 LRS statements will be downgraded to pass compatibility checks.
+As of v0.0.34 LRSPipe supports xAPI versions 1.0.3 and 2.0.0 using the `--source-xapi-version` and `--target-xapi-version` arguments. To pass validation checks LRSPipe must sometimes downgrade statement data to move it between LRS instances. Refer to the table below for more information:
+
+| version      | 1.0.3 target                  | 2.0.0 target |
+|--------------|-------------------------------|--------------|
+| 1.0.3 source | 2.0.0 data will be downgraded | no change    |
+| 2.0.0 source | 2.0.0 data will be downgraded | no change    |
+
+Note that even when using version 1.0.3 for both source and target (the default) downgrading may occur if 2.0.0 data is present on the 1.0.3 LRS. To ensure 2.0.0 data like `$.context.contextAgents` is always preserved use the `--target-xapi-version 2.0.0` argument.
 
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