[3.14] Docs: Fix markups for emphasis (GH-135598) (GH-135685)

Docs: Fix markups for emphasis (GH-135598)

The word emphasis character `_` is not supported as sphinx markup, so changed to `*`.
(cherry picked from commit 46c60e0d0b716e8e6f0b74a0f9d0542605b1efd4)

Co-authored-by: Yuki Kobayashi <drsuaimqjgar@gmail.com>