Accurate, concise documentation

Documentation (docstrings, inline comments, and changelog entries) must be accurate, concise, and maintainable:

• Describe actual behavior precisely. Avoid misleading wording (e.g. don’t say “a copy of the session is added” when the same object is reused — say what actually happens).
• Keep comments focused on why something is done, not on explaining language mechanics. E.g. prefer # make error available outside except block (py3) over a long paragraph re-explaining Python 3 exception scoping.
• Avoid duplicating information that lives elsewhere and can drift out of sync (e.g. don’t copy a dict of default values into a docstring — point readers to the attribute, like client.environ_base, instead).
• Use the project’s established documentation conventions consistently. For module/class-level attributes in this codebase, use #: comments above the attribute rather than dangling docstrings, since this is the existing convention and renders correctly in Sphinx.
• Put explanatory text in the right place: move behavioral notes out of stray comments and into the docstring of the relevant function/property.
• Only add .. versionadded:: / .. versionchanged:: directives and changelog entries for changes that affect the public API. Internal/private helpers (e.g. _get_debug, explain_template_loading_attempts) are not public-facing and should not carry user-facing version history.

Example of a good attribute doc:

#: Whether debug mode is enabled. When using ``flask run`` to start the
#: development server, an interactive debugger will be shown for
#: unhandled exceptions, and the server will be reloaded when code
#: changes.
debug = ConfigAttribute("DEBUG")

versus a comment that should instead be a docstring on the getter, or a versionchanged note that should be dropped entirely for a private helper.</description> </invoke>