[DOCS] Fix painless context _id variable (#101316)

docs/painless/painless-contexts/painless-reindex-context.asciidoc

@@ -19,7 +19,7 @@ reindexed into a target index.
 {ref}/mapping-index-field.html[`ctx['_index']`] (`String`)::
         The name of the index.
 
-{ref}/mapping-id-field.html[`ctx['_id']`] (`int`, read-only)::
+{ref}/mapping-id-field.html[`ctx['_id']`] (`String`)::
         The unique document id.
 
 `ctx['_version']` (`int`)::

docs/painless/painless-contexts/painless-update-by-query-context.asciidoc

@@ -20,7 +20,7 @@ result of query.
 {ref}/mapping-index-field.html[`ctx['_index']`] (`String`, read-only)::
         The name of the index.
 
-{ref}/mapping-id-field.html[`ctx['_id']`] (`int`, read-only)::
+{ref}/mapping-id-field.html[`ctx['_id']`] (`String`, read-only)::
         The unique document id.
 
 `ctx['_version']` (`int`, read-only)::

docs/painless/painless-contexts/painless-update-context.asciidoc

@@ -18,7 +18,7 @@ add, modify, or delete fields within a single document.
 {ref}/mapping-index-field.html[`ctx['_index']`] (`String`, read-only)::
         The name of the index.
 
-{ref}/mapping-id-field.html[`ctx['_id']`] (`int`, read-only)::
+{ref}/mapping-id-field.html[`ctx['_id']`] (`String`, read-only)::
         The unique document id.
 
 `ctx['_version']` (`int`, read-only)::