doc: document that sync.Pool should be used only with pointer types

[rasky]

To avoid spurious allocations, sync.Pool should always be used with pointers. For instance, when storing buffers as slices, a pointer to slice should be used instead of the slice itself. This was brought up in http://golang.org/cl/24371 by @dvyukov, but it's not documented.

Since the primary goal of sync.Pool is to reduce allocations, and using it with non-pointers makes it de-facto useless, it should be prominently documented to avoid using non-pointers with it.

[rsc]

I disagree that it's de-facto useless to have a pool of non-pointers. If you are saving multi-kilobyte slices, you may not care at all if each pool operation happens to allocate one pointer. This is an optimization detail, not something to expose to everyone reading the sync.Pool docs. If it's a problem in a real program, profiling will show that.