[DOC]Update doc in _api_internal.py and ir_pass.py (#2514)

python/tvm/_api_internal.py

-"""namespace of internal API"""
+"""Namespace of internal API
+
+The functions in this namespace are automatically exported from C++ side via PackedFunc
+that is registered by "TVM_REGISTER_*" macro. This way makes calling Python functions from C++
+side very easily.
+
+Each string starts with "_" in the "TVM_REGISTER_*" macro is an internal API. You can find
+all the functions in "api_lang.cc", "api_base.cc", "api_arith.cc" and "api_ir.cc" under "src/api".
+"""

python/tvm/ir_pass.py

@@ -4,7 +4,8 @@ This namespace is used for developers. While you do not see any declarations.
 The functions are automatically exported from C++ side via PackedFunc.
 
 Each api is a PackedFunc that can be called in a positional argument manner.
-You can read "include/tvm/pass.h" for the function signature of these functions.
+You can read "include/tvm/ir_pass.h" for the function signature and
+"src/api/api_pass.cc" for the PackedFunc's body of these functions.
 """
 from ._ffi.function import _init_api