rpc.tlsservd: Update the man page for the -C option

Commit 712aac1389e8 adds a new -C command line option
to rpc.tlsservd.

This patch updates the man page for this new command
line option.

This is a content change.

(cherry picked from commit d94358e29d1eacab17e9992ad91decb1b84b9449)