[3.12] gh-106033: [docs] Improve C API GetItem & HasAttr notes. (GH-106047) (#106070)

gh-106033: [docs] Improve C API GetItem & HasAttr notes. (GH-106047)

Use a note:: tag so that these dict and object API deficiencies show up clearly.

A caution:: tag was considered, but our current python docs rendering doesn't do much with that (no box or color change).  warning:: seemed too extreme.  note looks good.
(cherry picked from commit 19d6511b0b8f3f74e668ae32ccef89bcbf1a8a62)

Co-authored-by: Gregory P. Smith <greg@krypto.org>