Merge r1531009 from trunk:

doc improvements for brigade insertion macros

PR: 55115
Submitted by: Mike Rumph <mike.rumph oracle.com>
Reviewed by: trawick


git-svn-id: https://svn.apache.org/repos/asf/apr/apr-util/branches/1.5.x@1531010 13f79535-47bb-0310-9956-ffa450edef68