Fix https://issues.apache.org/bugzilla/show_bug.cgi?id=57331
Allow ExpiresFilter to use "year" as synonym for "years" in its configuration.
Update documentation to mention both "<unit>" and "<unit>s" duration unit names.
Fix minor formatting issue (Many lines in XML text were wrongly wrapped. because quote character was replaced by &#27; and that broke wrapping algorithm.)

Merged r1643963 from tomcat/trunk.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/tc8.0.x/trunk@1643964 13f79535-47bb-0310-9956-ffa450edef68