Action Docs/README out of sync and partially outdated

[tooomm]

There are several actions in this repo:

• actions/cache <-- main one
• actions/cache/restore <-- dedicated "load" part for more control
• actions/cache/save <-- dedicated "save" part for more control

The documentations in the different README files have conflicting statements and leads to confusion if one is building more granular flows and is using the restore+save actions.

Looking at the "outputs" section in restore README, it describes cache-hit differently than the main README.
That leads to quite some headaches when working with the actions.

• actions/cache/restore (https://github.com/actions/cache/blob/main/restore/README.md#outputs)
  
• actions/cache (https://github.com/actions/cache#outputs)
  

Note the difference on cache miss, one says it will return false, the other states it will return ''.

Users can also not be sure if cache-primary-key and cache-matched-key are deprecated and cache/restore docs are not updated, or if it's a unique feature only available on the "split" action.

I saw several issues around cache-hit behavior, as well as some PR's that tried to fix the documentation - all adding to the confusion. Apparently there were some changes on the logic (which got reverted again?) and the docs fall out of sync.

What I found:

• The main README yields the correct, current behaviour 👍
• Several issues and PRs are outdated and got not closed when the main docs got updated
• The restore README got not updated and has outdated information