[latexrefman] Stretch the interword space in @var{text} across the entire @var{width}.

[Karl Berry]

    Arabic ... the microtype package …

I'm not sure if microtype takes effect inside an \hbox to <width>.

In any case, such things affect typesetting globally. It would not be
practical, or useful, to say in every possible case "except when
microtype is loaded, or when Arabic/etc. scripts are in use, or this
other case, or that other case".

latex2e.texi is about standard TeX typesetting with non-elaborate
(alphabetic) scripts. We can't cover Arabic (or Urdu or Chinese or ...)
typesetting, which have a radically different basis.

Pur another way, virtually all statements in the document about
typesetting would not be universally true with microtype loaded, or when
applied to certain OpenType fonts, or (along another dimension) with
LuaTeX callbacks in place. Those things could be mentioned somewhere
globally, but are otherwise beyond the scope of our document, as far as
I can see. -k