[Docs] Improve tuning for speed advice (#33315)

This change merges two sections in the "Tune for search speed" documentation
that recommend mapping numeric identifiers as keywords. Both sections contain
mostly the same advice, so they can be merged.

Closes #32733

Author: Christoph Büscher

docs/reference/how-to/search-speed.asciidoc

@@ -165,12 +165,15 @@ GET index/_search
 // TEST[continued]
 
 [float]
-=== Mappings
+=== Consider mapping identifiers as `keyword`
 
 The fact that some data is numeric does not mean it should always be mapped as a
-<<number,numeric field>>. Typically, fields storing identifiers such as an `ISBN`
-or any number identifying a record from another database, might benefit from
-being mapped as <<keyword,`keyword`>> rather than `integer` or `long`.
+<<number,numeric field>>. The way that Elasticsearch indexes numbers optimizes
+for `range` queries while `keyword` fields are better at `term` queries. Typically, 
+fields storing identifiers such as an `ISBN` or any number identifying a record 
+from another database are rarely used in `range` queries or aggregations. This is
+why they might benefit from being mapped as <<keyword,`keyword`>> rather than as 
+`integer` or `long`.
 
 [float]
 === Avoid scripts
@@ -349,15 +352,6 @@ WARNING: Loading data into the filesystem cache eagerly on too many indices or
 too many files will make search _slower_ if the filesystem cache is not large
 enough to hold all the data. Use with caution.
 
-[float]
-=== Map identifiers as `keyword`
-
-When you have numeric identifiers in your documents, it is tempting to map them
-as numbers, which is consistent with their json type. However, the way that
-Elasticsearch indexes numbers optimizes for `range` queries while `keyword`
-fields are better at `term` queries. Since identifiers are never used in `range`
-queries, they should be mapped as a `keyword`.
-
 [float]
 === Use index sorting to speed up conjunctions