clarify docs

Applications can also use SSL_CTX_cmd() to process command lines though the

utility function SSL_CTX_cmd_argv() is normally used instead. One way

to do this is to check for an initial prefix ("-", "--" or "--ssl-" for example)

on a command argument and pass the rest to B<cmd>. The following argument

is passed to B<value> (which may be NULL).

to do this is to set the prefix to an appropriate value using

SSL_CONF_CTX_set1_prefix(), pass the current argument to B<cmd> and the

following argument to B<value> (which may be NULL).

In this case if the return value is positive then it is used to skip that

number of arguments as they have been processed by SSL_CTX_cmd(). If -2 is