Add back documentation on disable _source

[ppf2]

Currently, our 1.x doc (https://www.elastic.co/guide/en/elasticsearch/reference/1.x/mapping-source-field.html#mapping-source-field) includes information on:

• Ability to disable _source
• includes/excludes functionality for _source being deprecated in 1.6.0

In #10915, we removed the ability to disable _source, and then it was subsequently added back in #11171. But it appears that when we added this back, we haven't gone back and updated the documentation to re-add the information on disabling _source and the includes/excludes usage. So the latest doc in master is missing this information:

https://www.elastic.co/guide/en/elasticsearch/reference/master/mapping-source-field.html#include-exclude (doc in master)

https://github.com/elastic/elasticsearch/commits/d32a80f37b1e09d37b1556ba5cce74d0b76bb8c4/docs/reference/mapping/fields/source-field.asciidoc (history of doc changes)

This is a request to add back the documentation on disabling source and the caveats mentioned here (#11116 (comment)) now that the feature has returned. And since the feature is back, I think it also means that includes/excludes is no longer "deprecated"?

[ppf2]

thx! @clintongormley