fix: metadata schema & requirements (#274)

## What Did You Change?

Most of the content in "Metadata Schema and Requirements", which was
reported incorrect and fixed by @mesteva.

## Do You Want Support (syntax, design, etc)?

Yes, make sure I didn't break anything. I am also double checking.

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## Any Reference Material Worth Sharing?

[internal collab
doc](https://docs.google.com/document/d/1a1KSeyezPCwjDIHBSP2EEwauWFUKy_RiVVqKVepUAUA/#heading=h.fs1mjdamsvw3)

Author: wesleyboar