doc/go1.18: document FreeBSD 11.x will no longer be supported

Mention future versions will require the COMPAT_FREEBSD12 kernel option
set in the kernel.

For #47694.

Change-Id: Ia94c4f9dbb38c68025d3c1d12dd2e241a5480a6b
Reviewed-on: https://go-review.googlesource.com/c/go/+/354971
Reviewed-by: Ian Lance Taylor <[email protected]>
Reviewed-by: Tobias Klauser <[email protected]>

Author: paulzhol

doc/go1.18.html

@@ -31,8 +31,11 @@ <h2 id="language">Changes to the language</h2>
 
 <h2 id="ports">Ports</h2>
 
-<p>
-  TODO: complete this section, or delete if not needed
+<p id="freebsd">
+  Go 1.18 is the last release that is supported on FreeBSD 11.x, which has
+  already reached end-of-life. Go 1.19 will require FreeBSD 12.2+ or FreeBSD
+  13.0+.
+  FreeBSD 13.0+ will require a kernel with the COMPAT_FREEBSD12 option set (this is the default).
 </p>
 
 <h2 id="tools">Tools</h2>