[deque.modifiers] Clarify complexity requirements on deque insertion

[deque.modifiers]/2 states:

Complexity: The complexity is linear in the number of elements inserted plus the lesser of the distances to the beginning and end of the deque. Inserting a single element at either the beginning or end of a deque always takes constant time and causes a single call to a constructor of T.

This is misleading since all real-world implementations of deque are only amortized constant time when inserting at the end, due to the need to grow (hopefully exponentially) the mapping array. I think there are a few possible solutions here, some of which make sense to combine:

• Replace "takes constant time and" with "only" since the "single call to the constructor of T" is redundant with stating constant time in the strict interpretation of [container.requirements.pre]/2. This avoids the use of the misleading phrase.
• Add a cross-reference to [container.requirements.pre] when stating it is constant time to give readers proper context.
• Add "amortized" before "constant time"
• Explicitly state that an amortized constant number of operations on pointers may take place. Since this is observable via the use of allocator-derived fancy-pointers, it is reasonable for this to be a normative requirement.