rc.conf.5: clarify source directories

[fernape]

Summary

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

Test Plan

Apply patch and check resulting man page.

.Dd to be bumped at commit time.

[bcr]

Looks good. Don't forget the .Dd bump.

[fernape]

Thanks for the review!