Add some notes on build-time optimizations for TCMalloc.
Add some notes on build-time optimizations for TCMalloc.

This centrally enumerates:
* Static linking (not mentioned elsewhere)
* Sized delete (mentioned briefly in reference.md)
* Relaxed alignment (mentioned briefly in design.md)
* ABSL_ALLOCATOR_NOTHROW (mentioned briefly in reference.md)

PiperOrigin-RevId: 308854074
Change-Id: I2e1b53e69a6cc15562339960442e9bb2772a3beb

google/tcmalloc,google/tcmalloc,google/tcmalloc