In the manual sysctl (at least in lamp Debian) it is written:
> sysctl is used to modify kernel parameters at runtime. The parameters available are those listed under /proc/sys/
> Procfs is required for sysctl support in Linux.
That is, the key sysctl in Linux correspond to files in /proc/sys (which, incidentally, pleases me). The FreeBSD sysctl to work is used the same system call. Thus the list of keys in linux you can see, making find /proc/sys -type f (or not? please correct if wrong)
And then look at man proc, there is a (scanty) description of some parameters. You can see for example here: tldp.org/LDP/Linux-Filesystem-Hierarchy/html/proc.html
Other parameters that are in /proc/sys, but there is no documentation, the easiest way is probably to Google.