Increase number of available parsers in the Velocity pool

When lots of users access an xwiki instance you'll see the following in the logs:

INFO  o.x.v.i.DefaultVelocityEngine  - Runtime : ran out of parsers. Creating a new one.  Please increment the parser.pool.size property. The current value is too small. 

This is innocuous (except that it slows down a bit the velocity parsing) but would still be nicer to do something about it. 2 solutions (not exclusive):

• Increase the default value (which is 20)
• Document on xwiki.org how to increase the value (and make it possible)

FTR this is how it works:

• http://velocity.apache.org/engine/devel/developer-guide.html (search for "parser.pool.size")