Fix doc errors in APR header files.

PR: 55133
Submitted by: Mike Rumph <mike.rumph oracle.com>

Backport of r1496407 from trunk.


git-svn-id: https://svn.apache.org/repos/asf/apr/apr/branches/1.5.x@1528850 13f79535-47bb-0310-9956-ffa450edef68