Document multi-component profile paths

In env_variables.rst and krb5_conf.rst, document that KRB5_CONFIG can
contain multiple colon-separated pathnames.

(cherry picked from commit 28dc817d2a065f5e0ab73377350a0529b2f4aa48)

https://github.com/krb5/krb5/commit/d170ed6e57e5d3ebe0cf158b1bb6782bbcd74798
Author: Greg Hudson <ghudson@mit.edu>
Committer: Tom Yu <tlyu@mit.edu>
Commit: d170ed6e57e5d3ebe0cf158b1bb6782bbcd74798
Branch: krb5-1.13
 doc/admin/conf_files/krb5_conf.rst |    4 +++-
 doc/admin/env_variables.rst        |    5 +++--
 2 files changed, 6 insertions(+), 3 deletions(-)