Document multi-component PKINIT client certs

In pkinit.rst, note that the extensions.client file only works for
single-component client principals, and describe how to modify it for
multi-component principals.

https://github.com/krb5/krb5/commit/8abbb9b805e457849e9e414bd2ef610ad9fc4f06
Author: Greg Hudson <ghudson@mit.edu>
Commit: 8abbb9b805e457849e9e414bd2ef610ad9fc4f06
Branch: master
 doc/admin/pkinit.rst |   21 ++++++++++++++++++---
 1 files changed, 18 insertions(+), 3 deletions(-)