parser: Export Parser class as unstable API

Replicates graphql/graphql-js@a9a095e

Author: Cito

src/graphql/language/parser.py

@@ -157,6 +157,16 @@ def parse_type(
 
 
 class Parser:
+    """GraphQL AST parser.
+
+    This class is exported only to assist people in implementing their own parsers
+    without duplicating too much code and should be used only as last resort for cases
+    such as experimental syntax or if certain features couldn't be contributed upstream.
+
+    It's still part of the internal API and is versioned, so any changes to it are never
+    considered breaking changes. If you still need to support multiple versions of the
+    library, please use the `__version_info__` variable for version detection.
+    """
 
     _lexer: Lexer
     _no_Location: bool