pg_upgrade --logfile option documentation

The documentation of the pg_upgrade -l/--logfile option never made much
sense to me:
 -l, --logfile=FILENAME        log session activity to file

I don't know what "session" means for pg_upgrade, so I never used it.

What it actually does is log the output of all the programs that
pg_upgrade calls internally, such as pg_ctl, psql, vacuumdb,
pg_resetxlog, to the specified file, which is quite useful for analyzing
errors such as

unable to connect to new postmaster started with the command: "/usr/lib/postgresql/9.1/bin/pg_ctl" -w -l "/dev/null" -D
"/var/lib/postgresql/9.1/main"-o "-p 5433 -b" start >> "/dev/null" 2>&1
 

where -l would have put something in the place of /dev/null.

So what might be a better wording for this option?  Something like "log
output of internally called programs to file"?