Document JspAttribute.isDynamic() and correct wrong value of this flag for "omit" attribute of <jsp:attribute> action.

I think there were no observable consequences of this typo. The "dynamic" flag makes sense only for attributes of a CustomTag.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1590801 13f79535-47bb-0310-9956-ffa450edef68