[DOCS] Clean up ILM attributes

[lcawl]

The https://raw.githubusercontent.com/elastic/elasticsearch/master/docs/reference/ilm/index.asciidoc file currently defines the following attributes:

:ilm: index lifecycle management
:Ilm: Index lifecycle management
:ILM: ILM

All of those are considered to be the same value by the doc build, however, so all of them are output as the last definition in this list (ILM). For example, see https://www.elastic.co/guide/en/elasticsearch/reference/master/index-lifecycle-management.html

We need to define unique keywords for each of those attributes (ideally in the shared elastic/docs list of attributes, so that they can be used in other books too).

[elasticmachine]

Pinging @elastic/es-core-features

[lcawl]

Added ilm attributes in https://github.com/elastic/docs/blob/master/shared/attributes.asciidoc