Use infinities for one-sided ranges for a11y.

This was a request from Chrome to be able to specify
one-sided ranges. Rather than add new behavior, I'm just
documenting what seems to me a reasonable convention.

Bug: 30025621
Test: No tests as this just adds comments.
Change-Id: I1467f57729eed08a981c3e6d50951e78c3564a50