[Doc] libSyntax: add a document describing the status of libSyntax no…

…de specialization. NFC

This document breaks all existing syntactic AST nodes into five categories:
expression, declaration, statement, pattern and typerepr. Nodes in each
category can be further defined as "specialized", "in-progress" and
"not-specialized".

The parser currently can recognize specialized nodes; for "in-progress"
and "not-specialized" nodes, the parser will wrap their underlying source
to be an unknown entity, e.g. UnknownDecl or UnknownStmt.

"in-progress" nodes are considered in a higher priority to be specialized
than "not-specialized" nodes. However, eventually all nodes should be
marked as "specialized".