Apparently, I was too lazy to read to the end of the documentation… Further down, the documentation states:
subquery A subquery is used only in conjunction with the XML keyword. When you specify a subquery, all values found by the subquery are used for pivoting. […]
This will work
PIVOT XML (AVG(salary) FOR (company) IN (SELECT DISTINCT company FROM companies))
See the full documentation
http://docs.oracle.com/cd/B28359_01/server.111/b28286/statements_10002.htm#CHDFAFIE