[Mappings editor] add descriptions for data types

[alisonelizabeth]

This PR adds descriptions for keyword, dense_vector, completion, token_count, alias, search_as_you_type, and flattened data types.

Screenshots

[elasticmachine]

Pinging @elastic/es-ui (Team:Elasticsearch UI)

[cjcenizal]

LGTM!

[gchaps]

Alias fields accept an alternative name for a field. You can use aliases in search requests.

Alias fields accept an alternative name for a field, which you can use in search requests.

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Can this field be renamed Autocomplete or Word completion instead?

Not sure what the last half of the second sentence means

Completion suggester fields support autocomplete. This data type enables fast lookups, but is costly to build and store in memory.

Completion suggester fields support autocomplete, which enables fast lookups, but is costly to build and store in memory.

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Dense vector fields store vectors of float values. This data type is useful for document scoring.

Dense vector fields store vectors of float values, useful for document scoring.

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Flattened fields map a field as a single object. This data type is useful for indexing objects with a large or unknown number of unique keys. Flattened fields support basic queries only.

Flattened fields map a field as a single object and are useful for indexing objects with a large or unknown number of unique keys. Flattened fields support basic queries only.

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Search-as-you-type fields break strings into subfields for search suggestions. This data type supports both prefix and infix completion.

Question: Is "infix" a typo? Should it be in command font? Or is there another word you can use?

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Token count fields accept string values and indexes the number of tokens in the string.

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Keyword fields support searching for an exact value and are useful for filtering, sorting, and aggregations. To index full-text content, such as an email body, use the text data type.

Keyword fields support searching for an exact value and are useful for filtering, sorting, and aggregations.

Is the last sentence needed?

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[cjcenizal]

Merging this now to expedite other PRs. I'll address Gail's feedback in a separate PR.