Warn against pre-processing text with arabic_reshaper and python-bidi on Raqm builds - #10408

Adds the warning proposed in #9907 to the libraqm bullet in the building-from-source docs, where the bidirectional/shaping sentence quoted in the issue lives.

On a Raqm build, Pillow already performs bidirectional reordering (FriBiDi) and shaping (HarfBuzz). The widely-copied recipe of pre-applying arabic_reshaper and python-bidi — still necessary on builds without Raqm — therefore processes the text twice: it renders incorrectly, and the reshape step itself rewrites the string (lam-alef merged into a single presentation form, diacritics dropped). The new bullet states this and points readers at the conditional form:

if not features.check("raqm"):
    text = get_display(arabic_reshaper.reshape(text))

Docs-only change, one bullet added, no other files touched. Happy to move or reword the note if another location (e.g. the features reference or the text-drawing docs) is preferred.