[mypyc] Update docstrings of IR builder classes (#18246)

Having the documentation in class docstrings makes it easier to find.

mypyc/irbuild/builder.py

@@ -1,14 +1,6 @@
-"""Builder class used to transform a mypy AST to the IR form.
+"""Builder class to transform a mypy AST to the IR form.
 
-The IRBuilder class maintains transformation state and provides access
-to various helpers used to implement the transform.
-
-The top-level transform control logic is in mypyc.irbuild.main.
-
-mypyc.irbuild.visitor.IRBuilderVisitor is used to dispatch based on mypy
-AST node type to code that actually does the bulk of the work. For
-example, expressions are transformed in mypyc.irbuild.expression and
-functions are transformed in mypyc.irbuild.function.
+See the docstring of class IRBuilder for more information.
 """
 
 from __future__ import annotations
@@ -154,6 +146,30 @@ class UnsupportedException(Exception):
 
 
 class IRBuilder:
+    """Builder class used to construct mypyc IR from a mypy AST.
+
+    The IRBuilder class maintains IR transformation state and provides access
+    to various helpers used to implement the transform.
+
+    mypyc.irbuild.visitor.IRBuilderVisitor is used to dispatch based on mypy
+    AST node type to code that actually does the bulk of the work. For
+    example, expressions are transformed in mypyc.irbuild.expression and
+    functions are transformed in mypyc.irbuild.function.
+
+    Use the "accept()" method to translate individual mypy AST nodes to IR.
+    Other methods are used to generate IR for various lower-level operations.
+
+    This class wraps the lower-level LowLevelIRBuilder class, an instance
+    of which is available through the "builder" attribute. The low-level
+    builder class doesn't have any knowledge of the mypy AST. Wrappers for
+    some LowLevelIRBuilder method are provided for convenience, but others
+    can also be accessed via the "builder" attribute.
+
+    See also:
+     * The mypyc IR is defined in the mypyc.ir package.
+     * The top-level IR transform control logic is in mypyc.irbuild.main.
+    """
+
     def __init__(
         self,
         current_module: str,

mypyc/irbuild/ll_builder.py

@@ -1,11 +1,7 @@
 """A "low-level" IR builder class.
 
-LowLevelIRBuilder provides core abstractions we use for constructing
-IR as well as a number of higher-level ones (accessing attributes,
-calling functions and methods, and coercing between types, for
-example). The core principle of the low-level IR builder is that all
-of its facilities operate solely on the IR level and not the AST
-level---it has *no knowledge* of mypy types or expressions.
+See the docstring of class LowLevelIRBuiler for more information.
+
 """
 
 from __future__ import annotations
@@ -224,6 +220,22 @@
 
 
 class LowLevelIRBuilder:
+    """A "low-level" IR builder class.
+
+    LowLevelIRBuilder provides core abstractions we use for constructing
+    IR as well as a number of higher-level ones (accessing attributes,
+    calling functions and methods, and coercing between types, for
+    example).
+
+    The core principle of the low-level IR builder is that all of its
+    facilities operate solely on the mypyc IR level and not the mypy AST
+    level---it has *no knowledge* of mypy types or expressions.
+
+    The mypyc.irbuilder.builder.IRBuilder class wraps an instance of this
+    class and provides additional functionality to transform mypy AST nodes
+    to IR.
+    """
+
     def __init__(self, errors: Errors | None, options: CompilerOptions) -> None:
         self.errors = errors
         self.options = options