Enhance WT_CURSOR::reserve documentation around commit visibility

Our current documentation for WT_CURSOR::reserve doesn't explain that on commit the transaction no longer references the reserved slot, and so other transactions with this transaction in their snapshot will no longer conflict with the reserve operation.

i.e: the WT_UPDATE structure that was created as part of the reserve operation has it's transaction ID set to WT_TXN_NONE on transaction commit.