improve documentation of the service signatures
improve documentation of the service signatures

jpfr/open62541,JGrothoff/open62541,open62541/open62541,jpfr/open62541,StalderT/open62541,StalderT/open62541,open62541/open62541,StalderT/open62541,jpfr/open62541,jpfr/open62541,JGrothoff/open62541,StalderT/open62541,JGrothoff/open62541,open62541/open62541,open62541/open62541,JGrothoff/open62541