[patch] Improve documentation around FreeBSD Kernel Tuning

Hi,

I have a patch that improves the documentation for FreeBSD Kernel Tuning:

- Show a # prompt instead of $ to indicate a root shell is needed
- Remove the -w flag to sysctl since it is not needed anymore and just silently ignored
- Encourage the user to set the read-only sysctls in /boot/loader.conf, instead of setting them manually in the
loader.

I have put these in a github fork of the repo, but I am new to git. So I apologize if this is incorrect. 

https://github.com/so14k/postgres/commit/12c03bdb2967346e7ad9ce0bdd3db8dfcf81507e


Regards,
Brad Davis