Improve builder API doc

doc/extdev/builderapi.rst

@@ -16,7 +16,7 @@ Builder API
 
    .. rubric:: Overridable Attributes
 
-   These attributes should be set on builder sub-classes:
+   These class attributes should be set on builder sub-classes:
 
    .. autoattribute:: name
    .. autoattribute:: format
@@ -29,8 +29,7 @@ Builder API
 
    .. rubric:: Core Methods
 
-   These methods are predefined and should generally not be overridden,
-   since they form the core of the build process:
+   These methods are define the core build workflow and cannot be overridden:
 
    .. automethod:: build_all
    .. automethod:: build_specific
@@ -70,9 +69,10 @@ Builder API
 
    .. rubric:: Overridable Attributes (extensions)
 
-   Builder sub-classes can set these attributes to support built-in extensions:
+   Builder sub-classes can set these class attributes to support built-in extensions:
 
    .. attribute:: supported_linkcode
+      :type: str
 
       By default, the :mod:`linkcode <sphinx.ext.linkcode>` extension will
       only inject references for an ``html`` builder.

sphinx/builders/__init__.py

@@ -59,33 +59,38 @@ class Builder:
     Builds target formats from the reST sources.
     """
 
-    #: The builder's name, for the -b command line option.
-    name = ''
+    #: The builder's name. This is the name used for the
+    #: :option:`sphinx-build -b` command line option.
+    name: str = ''
     #: The builder's output format, or '' if no document output is produced.
-    format = ''
+    #: This is commonly the file extension, e.g. "html", but can technically
+    #: be any string. Its value can be used by various components such as
+    #: :class:`.SphinxPostTransform` or extensions to determine whether they
+    #: can work with the builder.
+    format: str = ''
     #: The message emitted upon successful build completion. This can be a
     #: printf-style template string with the following keys: ``outdir``,
     #: ``project``
-    epilog = ''
+    epilog: str = ''
 
     #: default translator class for the builder.  This can be overridden by
     #: :py:meth:`~sphinx.application.Sphinx.set_translator`.
     default_translator_class: type[nodes.NodeVisitor]
     # doctree versioning method
     versioning_method = 'none'
     versioning_compare = False
-    #: allow parallel write_doc() calls
-    allow_parallel = False
+    #: Whether it is safe to make parallel :meth:`~.Builder.write_doc()` calls.
+    allow_parallel: bool = False
     # support translation
     use_message_catalog = True
 
     #: The list of MIME types of image formats supported by the builder.
     #: Image files are searched in the order in which they appear here.
     supported_image_types: list[str] = []
     #: The builder can produce output documents that may fetch external images when opened.
-    supported_remote_images = False
+    supported_remote_images: bool = False
     #: The file format produced by the builder allows images to be embedded using data-URIs.
-    supported_data_uri_images = False
+    supported_data_uri_images: bool = False
 
     def __init__(self, app: Sphinx, env: BuildEnvironment) -> None:
         self.srcdir = app.srcdir
@@ -159,7 +164,7 @@ def get_target_uri(self, docname: str, typ: str | None = None) -> str:
     def get_relative_uri(self, from_: str, to: str, typ: str | None = None) -> str:
         """Return a relative URI between two source filenames.
 
-        May raise environment.NoUri if there's no way to return a sensible URI.
+        Raise :exc:`!sphinx.errors.NoUri` if there's no way to return a sensible URI.
         """
         return relative_uri(
             self.get_target_uri(from_),
@@ -789,7 +794,16 @@ def copy_assets(self) -> None:
         pass
 
     def write_doc(self, docname: str, doctree: nodes.document) -> None:
-        """Where you actually write something to the filesystem."""
+        """
+        Write the output file for the document given by
+        :term:`docname <document name>`.
+
+        *doctree* defines the content to be written.
+
+        The output filename must be determined within this method, typically by
+        calling :meth:`~.Builder.get_target_uri` or
+        :meth:`~.Builder.get_relative_uri`.
+        """
         raise NotImplementedError
 
     def write_doc_serialized(self, docname: str, doctree: nodes.document) -> None: