how to list the attributes of various enumerations like severity, priority in rtc through rest query

You can use the discovery chain method to figure it out.
http://www.ibm.com/developerworks/rational/library/10/programmatic-authentication-and-certificate-handling-for-rational-team-concert-2-0/
In this document, it shows you how to get the work item services. From the output, locate the work item type that you are interested in, say, defect.

<oslc:creationFactory>
<oslc:CreationFactory>
<dcterms:title rdf:parseType="Literal">Location for creation of change requests</dcterms:title>
<oslc:resourceType rdf:resource="http://open-services.net/ns/cm#ChangeRequest"/>
<oslc:resourceShape rdf:resource="https://clm404.sydney.rcs:9443/ccm/oslc/context/_U-8CoCGlEeOpzLt8rZoBTA/shapes/workitems/defect"/>
<oslc:creation rdf:resource="https://clm404.sydney.rcs:9443/ccm/oslc/contexts/_U-8CoCGlEeOpzLt8rZoBTA/workitems"/>
</oslc:CreationFactory>
</oslc:creationFactory>

Follow the link of the resourceShape, and in this output you will see the enumerations for an attribute, such as

<rdf:Description rdf:about="https://clm404.sydney.rcs:9443/ccm/oslc/context/_U-8CoCGlEeOpzLt8rZoBTA/shapes/workitems/defect/property/internalSeverity/allowedValues">
<rdf:type rdf:resource="http://open-services.net/ns/core#AllowedValues"/>
<oslc:allowedValue rdf:resource="https://clm404.sydney.rcs:9443/ccm/oslc/enumerations/_U-8CoCGlEeOpzLt8rZoBTA/severity/severity.literal.l6"/>
<oslc:allowedValue rdf:resource="https://clm404.sydney.rcs:9443/ccm/oslc/enumerations/_U-8CoCGlEeOpzLt8rZoBTA/severity/severity.literal.l5"/>
<oslc:allowedValue rdf:resource="https://clm404.sydney.rcs:9443/ccm/oslc/enumerations/_U-8CoCGlEeOpzLt8rZoBTA/severity/severity.literal.l4"/>
<oslc:allowedValue rdf:resource="https://clm404.sydney.rcs:9443/ccm/oslc/enumerations/_U-8CoCGlEeOpzLt8rZoBTA/severity/severity.literal.l3"/>
<oslc:allowedValue rdf:resource="https://clm404.sydney.rcs:9443/ccm/oslc/enumerations/_U-8CoCGlEeOpzLt8rZoBTA/severity/severity.literal.l2"/>
<oslc:allowedValue rdf:resource="https://clm404.sydney.rcs:9443/ccm/oslc/enumerations/_U-8CoCGlEeOpzLt8rZoBTA/severity/severity.literal.l1"/>
</rdf:Description>

You can get the literal of the allowed values by following their respective links.