[3.12] C-API docs: Clarify the size of arenas (GH-110895) (#110946)

C-API docs: Clarify the size of arenas (GH-110895)

Clarify the size of arenas

From 3.10.0 alpha 7, the pymalloc allocator uses arenas with a fixed size of 1
MiB on 64-bit platforms instead of 256 KiB on 32-bit platforms.
(cherry picked from commit f07ca27709855d4637b43bba23384cc795143ee3)

Co-authored-by: Mienxiu <82512658+mienxiu@users.noreply.github.com>