Improve documentation of UA_DataSource
Improve documentation of UA_DataSource

This improves the documentation (doc-string) of the `typedef UA_DataSource`. The old documentation of `UA_DataSource` is incorrect: Some arguments were missing or have different names in the recent versions.
StalderT/open62541,StalderT/open62541,open62541/open62541,jpfr/open62541,JGrothoff/open62541,open62541/open62541,open62541/open62541,JGrothoff/open62541,StalderT/open62541,JGrothoff/open62541,open62541/open62541,jpfr/open62541,JGrothoff/open62541,jpfr/open62541,jpfr/open62541,StalderT/open62541