doc: clarify graph view docs to note negative values appear in profile comparison (#667)
doc: clarify graph view docs to note negative values appear in profile comparison (#667)

* doc: clarify graph view docs to note negative values appear in profile comparison.

Fixes https://github.com/google/pprof/issues/666

* 80 char col
google/pprof,google/pprof,google/pprof,google/pprof