entities card: secondary_info: explained the values

[MacDada]

Explained the meaning of last-change and last-updated values of secondary_info of Entities card.

I've stumbled upon the dilemma which one to use – this is the explanation I was given on the HA's discord.

Type of change

• [ ] Spelling, grammar or other readability improvements (current branch).
• [x] Adjusted missing or incorrect information in the current documentation (current branch).
• [ ] Added documentation for a new integration I'm adding to Home Assistant (next branch).
  • [ ] I've opened up a PR to add logos and icons in Brands repository.
• [ ] Added documentation for a new feature I'm adding to Home Assistant (next branch).
• [ ] Removed stale or deprecated documentation.

Checklist

• [x] This PR uses the correct branch, based on one of the following:
  • I made a change to the existing documentation and used the current branch.
  • I made a change that is related to an upcoming version of Home Assistant and used the next branch.
• [x] The documentation follows the Home Assistant documentation standards.

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[frenck]

Because there hasn't been any activity on this PR for quite some time now, I've decided to close it for being stale.

Feel free to re-open this PR when you are ready to pick up work on it again 👍