Improve Fields Documentation.
Write some fields definition examples.
Fix list type description.
@leplatrem review.
Better description of the metadata field.
Improve metadata example.
@leplatrem review.
Merge pull request #243 from spiral-project/improve-fields-documentation

Improve Fields Documentation.
Add a metadata field which isn't validated.
Add a test for the metadata field
Fix the test for the metadatafield.
Rename metadata to annotation.