Data lifecycle docs: Better explain rollovers, fix examples

[kilfoyle]

More feedback-based work for the Configure a lifecycle policy and related docs, specifically for rollovers:

• I've found my lifecycle policy and set it to X days in Hot phase, Y days in Warm phase, Z days in Cold phase. But it's not working.
  We need to document that:
  • Time is since last rollover
  • Users need to open Advanced settings > Enable rollover, change rollover settings (size, time). Details are in the Note on the “Phase transitions” page.
  • Maybe duplicate the note and link to “Phase transitions” from “Configure a lifecycle policy”.

Comments: I've added the note formatting for the section that mentions how time is calculated from rollover, similar to the Phase transitions page.

• In the “Create lifecycle policy” section, why does the example have zero days?
  This comment refers to an older version of the docs that used an older screenshot. Essentially it refers to the same point above about min_age being calculated from the time of rollover and not index creation time.

• What is rollover? This is documented but in a separate section from the configure a lifecycle docs. Expand on the concept of rollover in the lifecycle configuration docs, and link to the main rollover topic to learn more.

Comments: I've slightly reworded the link to the rollover lifecycle action doc, so that it's more accurate compared to the link to the Rollover page in the Index lifecycle management section.

• Why does rollover work differently on indices versus on data streams? Explain this in the rollover docs, and mention it in the ILM docs, data stream docs.
  This has been addressed by previous improvements to both pages: Rollover and Configure a lifecycle policy