Per gri's suggestion on CL 41623,
add a comment to trackAllTypes
about the trade-offs of enabling it.

Change-Id: Iec42b0da7933543200729003d1b2c6e0d9dcc5f0
Reviewed-on: https://go-review.googlesource.com/42186Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>