[3.15] Correct Stable ABI documentation for METH_FASTCALL (GH-149593) (GH-150853)

[3.15] Correct Stable ABI documentation for METH_FASTCALL (GH-149593) (GH-150853)

The current documentation says:

>
> METH_FASTCALL
>    Part of the Stable ABI since version 3.7.
>
> [...]
>
>   Added in version 3.7.
>
>    Changed in version 3.10: METH_FASTCALL is now part of the stable ABI.

so is contradictory about when it was added to the Stable ABI.  Looking at the header it seems like 3.10 is right.
(cherry picked from commit 58beae7319c58d850184d621d6635de23f71a229)

Co-authored-by: da-woods <dw-git@d-woods.co.uk>