>> --- a/doc/emacs/text.texi
>> +++ b/doc/emacs/text.texi
>> @@ -1097,6 +1097,11 @@ Outline Format
>>  chapters.  This works as long as no other command starts with
>>  @samp{@@chap}.
>>
>> +@vindex outline-search-function
>> +  Instead of setting the variable @code{outline-regexp}, you can set
>> +the variable @code{outline-search-function} to a function that
>> +matches the current heading and searches for the next one.
>
> This should tell which arguments will be passed to the function, and
> what it is expected to do and return.

This is the Emacs User Manual, not the Emacs Lisp Reference Manual.
Are you sure that arguments should be described here?

>> +(defvar-local treesit-outline-predicate nil
>> +  "Predicate used to find outline headings in the syntax tree.
>> +Intended to be set by a major mode.  When nil, the predicate
>> +is constructed from the value of `treesit-simple-imenu-settings'
>> +when a major mode sets it.")
>
> The doc string should explain with what arguments will the predicate
> be called and when it is expected to return non-nil.

Fixed.

>> +(defun treesit-outline-predicate-from-imenu (node)
>> +  "Return an outline searching predicate created from Imenu.
>> +Return the value suitable to set `treesit-outline-predicate'.
>> +Create this predicate from the value `treesit-simple-imenu-settings'
>> +that major modes set to find Imenu entries.  The assumption here
>> +is that the positions of Imenu entries most of the time coincide
>> +with the lines of outline headings.  When this assumption fails,
>> +you can directly set a proper value to `treesit-outline-predicate'."
>
> Likewise here: please describe in the doc string the signature and
> expectations from the predicate returned by this function (or point to
> the doc of some other variable/function where that information is
> included).

Actually, this is an internal function and doesn't need a docstring at all.
So I added two slashes to its name, and demoted its docstring to comments.

>> +(defun treesit-outline-search (&optional bound move backward looking-at)
>> +  "Search for the next outline heading in the syntax tree.
>> +See the descriptions of arguments in `outline-search-function'."
>
> What about the return value (if it is important)?

The return value is described in the docstring of
`outline-search-function' where the docstring above links to.