[inplace.vector] Move reserve+shrink_to_fit from #modifiers to #capacity

[Quuxplusone]

In the synopsis [inplace.vector.overview], reserve() and shrink_to_fit() are under size/capacity, but their actual descriptions are in [inplace.vector.modifiers], not [inplace.vector.capacity].

And the English title of [inplace.vector.capacity] doesn't quite match [vector.capacity], either. I think this is editorial and easy to patch.

[jensmaurer]

@jwakely , what do you think?

[jensmaurer]

@jwakely , ping.

[AlisdairM]

For reference, this PR triggered me, as I thought the equivalent wording was structured differently in vector. When I checked the PR against our existing text though, I was wrong and I concur that this is the better way.

[Quuxplusone]

Data point: I, also, think it's weird that resize isn't under "modifiers," and would be totally fine if someone went and fixed all the other containers to put resize, reserve, and shrink_to_fit under "modifiers" instead. :) But consistent one way or the other.