Assert that MappingData object is empty before performing mapping work.
It is backport of r1604663

Motivation: Remove dead branches. Protect Mapper from operating on
non-recycled MappingData. Using non-recycled MappingData might result in
mapping request onto a different target, like an issue that prevented us
from releasing 8.0.4 and fixed by r1580080/r1580083. I do not know such
bugs in Tomcat 6, but I want the code to be more safe.
Just a single (mappingData.host != null) check is enough to discern
recycled vs. non-recycled data. Checks for other MappingData fields are
removed by this patch, simplifying the code.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/tc6.0.x/trunk@1637679 13f79535-47bb-0310-9956-ffa450edef68
CTR: docs
Correct typos in configuration samples: XML comments start with '<!--'.
Remove comment markers around configuration samples

It is backport of r1446641 r1446660 from tomcat/tc7.0.x/trunk.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/tc6.0.x/trunk@1637681 13f79535-47bb-0310-9956-ffa450edef68