Update step `uses` description and link to docs

[ericsciple]

Today the step uses description misleads customers into thinking actions may only be defined in the same repo or in a public repo.

We should clarify this and also link to the docs.

For more context refer:

• https://github.com/orgs/community/discussions/26245#discussioncomment-15618657

[jsoref]

You should use community/community instead of orgs/community, the links are prettier:

• Path to action in the same repository as workflow community/community#26245 (comment)

(Or you could get someone to fix github's handling of org discussion links..., but where's the fun in fixing things for everyone?)