[6.2] [Docs] Add example for updating meta field (#35893)

[geekpete]

Backports the following commits to 6.2:

• [Docs] Add example for updating meta field (Add example for updating _meta field #35893)

[lcawl]

The failure of the docs checks in this branch are expected per https://github.com/elastic/infra/issues/8316

[lcawl]

When I run ./gradlew -p docs check locally, it fails with this error:

...
1> [2019-01-31T16:59:46,636][INFO ][o.e.s.DocsClientYamlTestSuiteIT] [test > {yaml=reference/mapping/fields/meta-field/line_9}]: before test
1> [2019-01-31T16:59:46,855][INFO ][o.e.s.DocsClientYamlTestSuiteIT] Stash dump on test failure [{
1> "stash" : {
1> "body" : {
1> "error" : {
1> "root_cause" : [
1> {
1> "type" : "illegal_argument_exception",
1> "reason" : "Rejecting mapping update to [my_index] as the final mapping would have more than 1 type: [_doc, user]",
1> "stack_trace" : "[Rejecting mapping update to [my_index] as the final mapping would have more than 1 type: [_doc, user]]; nested: IllegalArgumentException[Rejecting mapping update to [my_index] as the final mapping would have more than 1 type: [_doc, user]];

I get the same error when I try running the new example against Elasticsearch 6.2.5 locally.

[lcawl]

The book itself builds fine, however, using the docbldesold alias in https://github.com/elastic/docs/blob/master/doc_build_aliases.sh

[geekpete]

Looks like I need to adjust for the older version where the type should be /doc and not /_doc, so I'll fix that.

[lcawl]

LGTM and builds successfully (with docbldes alias)