Docs: some glossary terms

[PhaedrusTheGreek]

Some missing terminology:

merge - This term is used to describe the merging of Lucene segments in automatic background merging, or in a force merge.

reindex - To cycle through some or all documents in one or more indices, re-writing them into the same or new index in a local or remote cluster. This is most commonly done to update mappings, or to upgrade Elasticsearch between two incompatible index versions.

shrink - To reduce the amount of shards in an index. See the Shrink Index API

split - To grow the amount of shards in an index. See the Split Index API.

[elasticmachine]

Pinging @elastic/es-docs

[elasticmachine]

Pinging @elastic/es-core-infra

[Sue-Gallagher]

Questions:

1. It's never a good idea to use a word to define itself. Can we say:
   merge - The combining of Lucene segments, either automatically in the background or initiated using force merge.
2. You said:
   "...all documents in one or more indices, re-writing them into the same or new index..."
   We spend a lot of electrons telling our users that an index is immutable, so how does rewriting index data back into itself work?

[PhaedrusTheGreek]

1. Sounds better, yes
2. An in-place re-index is actually an simple _update_by_query, which is a shared code base with the reindex API - although it is one of those esoteric things. We could always add another sentence: "Reindexing to the same index is known as an Update By Query"

[Sue-Gallagher]

Fixed. See PR #66.