doc: 'timingSafeEqual' missing return type in v8.x

Compare the versions in both v8.x and v10.x：

v8.x:

v10.x:

I see the latest codes have included the 'return value', Maybe it should be also included into the v8.x version in the doc Apis?

Thanks anyway!

[richardlau]

Probably requires a backport of #19853.

[apapirovski]

Addressed in a PR.