[vector.capacity] vector::reserve is tutorial-like

[jwakely]

Effects: A directive that informs a vector of a planned change in size, so that it can manage the storage
allocation accordingly.

This is not a description of what the function does, it's a description of what the function is.

Maybe just strike it.

For hive Matt originally copied this text, but we're suggesting a different form using "If n <= capacity() there are no effects." and "Postconditions: capacity() >= n is true. But that would be a topic for an LWG issue. Striking the tutorial-like first sentence seems editorial.