doc: Fix description of error parameter for ssh_get_error*

ssh_get_error can actually work on anything with an ssh_common_struct
as its first member.  It is already used in examples in the
distribution with ssh_sessions and ssh_binds.

Signed-off-by: Alan Dunn <amdunn@gmail.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>