Improve documentation of sv_dup and sv_dup_inc

[Leont]

This is a follow-up on #22291

This reverts marking these functions as core-only. They really need to be API because it's often impossible to write thread-safe XS modules without them.

I've added an explanation of when exactly you need these functions, without that it's rather unclear when you'd need them (and when not).

This also clarifies that the pointer table is in fact not a cache, but an essential element of the cloning process. Without it, an non-incremented sv_dup wouldn't even make sense (something else would need to dup and increment it).

I haven't touched the last paragraph because that needs #23325 to be resolved first.

• This set of changes does not require a perldelta entry.