Tune for search speed documentation

[ahadadi]

In Tune for search speed there are two sections recommending to map numeric identifiers as keywords.
One is Mappings and the other is Map identifiers as keyword.
Only Map identifiers as keyword should be kept, or the sections can be merged.

[elasticmachine]

Pinging @elastic/es-search-aggs

[cbuescher]

@ahadadi thanks for this observation, I agree and opened #33315. Lets see what others think.