这是 Emacs 自带的功能，从 26.3 引入的，当时我就觉得做得太多了，一个 Tab 补全就加载，那个时候就关闭了。
(define-obsolete-variable-alias 'help-enable-completion-auto-load
'help-enable-completion-autoload "27.1")
(defcustom help-enable-completion-autoload t
"Whether completion for Help commands can perform autoloading.
If non-nil, whenever invoking completion for `describe-function'
or `describe-variable' load files that might contain definitions
with the current prefix. The files are chosen according to
`definition-prefixes'."
:type 'boolean
:group 'help
:version "26.3")