Minor doc changes to clarify mapping index param for string type (#22652)

* Grammatical correction

* Add note for legacy string mapping type

* Update truncate token filter to not mention the keyword tokenizer

The advice predates the existence of the keyword field

Closes #22650

docs/reference/mapping/params/index.asciidoc

@@ -4,3 +4,4 @@
 The `index` option controls whether field values are indexed. It accepts `true`
 or `false`. Fields that are not indexed are not queryable.
 
+NOTE: For the legacy mapping type <<string,`string`>> the `index` option only accepts legacy values `analyzed` (default, treat as full-text field), `not_analyzed` (treat as keyword field) and `no`.

docs/reference/mapping/types/string.asciidoc

@@ -12,7 +12,7 @@ to use `text` or `keyword`.
 
 Indexes imported from 2.x *only* support `string` and not `text` or `keyword`.
 To ease the migration from 2.x Elasticsearch will downgrade `text` and `keyword`
-mappings applied to indexes imported to 2.x into `string`. While long lived
+mappings applied to indexes imported from 2.x into `string`. While long lived
 indexes will eventually need to be recreated against 5.x before eventually
 upgrading to 6.x, this downgrading smooths the process before you find time for
 it.