The best way to find out what something means is to read the documentation. If you want to know what logFC is from toptable, then open the help page help("toptable") and scan down for the entry for logFC, which says:
"logFC: estimate of the log2-fold-change corresponding to the effect or contrast"
Every output quantity is carefully documented in this way.
What sources say that logFC is not the log2 fold-change?