dpg, add logics to get response's description when calculating return type's javadoc

[haolingdong-msft]

Our codegen does not have logic to get response's description, and only have the fall back logics to calculate from operation's desc and return type schema's desc. codegen code:

autorest.java/javagen/src/main/java/com/azure/autorest/mapper/ClientMethodMapper.java

Line 1589 in 8ee3658

protected static String returnTypeDescription(Operation operation, IType returnType, IType baseType) {

Try a pr, but has some issue handling ok response.
https://github.com/Azure/autorest.java/pull/2877/files

Note:
may need additional logics to format description.