I checked several configuration files, and found that the "plesklog" format, that Plesk uses to define the Apache LogFormat value, does store this information in the logfiles.
However, the Awstats model configuration file is not properly configured to retrieve this information from the logs.
By default, the value of the LogFormat parameter in /etc/awstats/awstats.model.conf will be "LogFormat=4", which is the Apache native common log format, not the combined, or plesklog format.
By changing this value to "LogFormat=1", these features (browsers, os, robots,...) will be shown in the statistics of your domain(s).
After modifying the awstats.model.conf file, you'll need to modify the awstats configuration files for your existing domains.
You can use this perl script to do so:
perl -e "s/LogFormat=4/LogFormat=1/g;" -pi $(find /opt/psa/etc/awstats/ -type f)
This will go through all the files inside the /opt/psa/etc/awstats/ directory, look for "LogFormat=4", and replace it by "LogFormat=1". Be sure to create a backup of all your files, in case something goes wrong.
The new statistics features will only be visible after the next calculation.