[#2621] Document delete-only ramp color

[CYX22222003]

Fixes #2621

Proposed commit message

Document delete-only ramp color.

Clarify that ramps containing only deleted lines are shown in red, while
ramps with both added and deleted lines continue to use the normal
time-period colors.

Other information

Docs-only change. Verified with git diff --check.

[damithc]

Thanks for the quick fix, @CYX22222003