rc.conf.5: clarify source directories

In 454adf0fa4d787bf6e6ff7be0aa6c27a91d2be8e some of the issues reported in the
PR where addressed. This commit adds a clarification about how the prefix of the
directories to be sourced actually behave.

PR: 197152
Reported by: jason.mann+freebsd@gmail.com
Approved by: manpages (bcr@)
Differential Revision: https://reviews.freebsd.org/D38627