for each tag in a namespace, is there easy way to get documentation on
what's that tag use for?

for Java, we have Java Doc, but for XML, do we usually refer to schema
file for the XML?

like XMLSchema name space, it has all the tags like
<xs:sequence>
<xs:element>
<xs:annotation>

where can I get more info for each tag?

===============
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
xmlns:sql="urn:schemas-microsoft-com:mapping-schema">