man: Add hints on using --pfs option
man: Add hints on using --pfs option

I ran "openconnect --pfs" to connect to 37 semi-randomly chosen servers:

  33 servers failed with a TLS fatal alert
   3 servers successfully negotiated the connection
   1 server no longer existed

According to Cisco, PFS support is about a year old in the 9.1 branch[1],
two years old in the 8.4 branch[2], and absent in 8.6/9.0.

So, if this means that some ~90% of users will not have much luck with
--pfs, we can at least offer some information to help their system
administrators configure it.

[1] http://www.cisco.com/c/en/us/td/docs/security/asa/asa91/release/notes/asarn91.html#pgfId-685480o
[2] http://www.cisco.com/c/en/us/td/docs/security/asa/asa84/release/notes/asarn84.html#pgfId-580804

Signed-off-by: Kevin Cernekee <3105dbb3dd63fbb4f6371cfb04adc6726f76c56b@gmail.com>

cernekee/openconnect,matlockx/openconnect,don-johnny/openconnect,nmav/openconnect-mine,neshema/openconnect,mveplus/openconnect-client,brandt/openconnect,mtmiller/openconnect,mtmiller/openconnect,mtmiller/openconnect,lins05/openconnect,don-johnny/openconnect,atyndall/openconnect,chadcatlett/openconnect,chadcatlett/openconnect,nmav/openconnect-mine,brandt/openconnect,brandt/openconnect,nmav/openconnect-mine,chadcatlett/openconnect,mveplus/openconnect-client,lins05/openconnect,mveplus/openconnect-client,mveplus/openconnect-client,atyndall/openconnect,liiir1985/openconnect,matlockx/openconnect,liiir1985/openconnect,cernekee/openconnect,lins05/openconnect,liiir1985/openconnect,don-johnny/openconnect,neshema/openconnect,lins05/openconnect,atyndall/openconnect,cernekee/openconnect,brandt/openconnect,atyndall/openconnect,mtmiller/openconnect,neshema/openconnect,matlockx/openconnect,nmav/openconnect-mine,cernekee/openconnect,chadcatlett/openconnect,liiir1985/openconnect,neshema/openconnect,matlockx/openconnect,don-johnny/openconnect