Reword javadocs on Inventory.contains(stack, amount) / containsAtLeast

I performed the 78-column formatting on just the javadocs of the two methods I touched.

#### Motivation
`contains(stack,int)` counts the **number of identical stacks** (NOT the sum of their counts) while at the same time also **matching on stack amount**.
Meanwhile, containsAtLeast() counts the amounts in the stacks, which is the common case (esp. now with ItemMeta).

#### Changes
 - A `@see` is added to `contains(stack, amount)` pointing to containsAtLeast().
 - The wording `"exactly matching"` is added to `contains(stack, amount)` as well as moving the first mention of amount earlier in the sentence.
 - The text on contains()'s @param amount is redone to be more clear.
 - The 'matches amount' disclaimer is redone.
 - the word 'any' ("contains any ItemStacks...") is removed from both javadocs, as we aren't looking for 'any', we're looking for 'at least'.
 - 'sum to the minimum amount specified' at the end of containsAtLeast()

This is basically documenting the the implementation, but it needs to be done.