Doc: C API: Move `tp_dealloc` paragraph to `tp_dealloc` section (#125737)

It looks like commit 43cf44ddcce6b225f959ea2a53e4817244ca6054
(gh-31501) accidentally moved the paragraph to the `tp_finalize`
section when the intent was to move it to the `tp_dealloc` section
(according to the commit message).

Also:
  * Convert the paragraph to a warning.
  * Apply the appropriate font style to `tp_dealloc`.
  * Unlinkify the first mention of `tp_dealloc` since the paragraph is
    already in the `tp_dealloc` section.