Update internals manual on how to add AST nodes.

StmtDumper.cpp is called ASTDumper.cpp these days, and usually works well
for new AST nodes without changes.  There's now DataRecursiveASTVisitor
in addition to RecursiveASTVisitor, and serialization wasn't mentioned.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212426 91177308-0d34-0410-b5e6-96231b3b80d8