Improve docs about runtime signalling. am: 31b44b3a70 am: 1afd8f6aaf am: 6c12f04ba1 am: 8754ceef79
Improve docs about runtime signalling. am: 31b44b3a70 am: 1afd8f6aaf am: 6c12f04ba1 am: 8754ceef79

Original change: https://android-review.googlesource.com/c/platform/external/perfetto/+/1454876

Change-Id: I0174c1620c915a92fce41f407649a9c074f253c8

google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto