doc: document Go 1.2.1

https://code.google.com/p/go/source/browse/doc/install.html?r=bbb657bcc92d1419bc2881c0b794666cd6861816
doc/install.html: FreeBSD 8 and higher only are supported

https://code.google.com/p/go/source/browse/doc/install.html?r=e2e0e740916bbf1d795adf2989d9b9851341b26b
doc: refer to wiki page for downloads

https://code.google.com/p/go/source/browse/doc/install.html?r=dcf1fdbf4686bfce9b637309cca8e4f2334205aa
doc/install.html: update wrt to OS and compiler support.

https://code.google.com/p/go/source/browse/doc/install.html?r=89509169b5a958382941ad1935f5bd4ece3ebb2e
doc: fix download link