[DOCS] EQL: Add search/index speed tip for functions (#54346)
[DOCS] EQL: Add search/index speed tip for functions (#54346)

EQL functions are an easy way for users to transform indexed data
at search time. However, using multiple functions can make
queries difficult to write and slows search speeds.

Users can circumvent this by indexing fields containing the transformed
data, but that usually slows index speeds.

This adds a related tip and example covering these tradeoffs.
uschindler/elasticsearch,uschindler/elasticsearch,nknize/elasticsearch,GlenRSmith/elasticsearch,HonzaKral/elasticsearch,nknize/elasticsearch,gingerwizard/elasticsearch,gingerwizard/elasticsearch,robin13/elasticsearch,uschindler/elasticsearch,robin13/elasticsearch,gingerwizard/elasticsearch,nknize/elasticsearch,GlenRSmith/elasticsearch,gingerwizard/elasticsearch,GlenRSmith/elasticsearch,robin13/elasticsearch,scorpionvicky/elasticsearch,robin13/elasticsearch,nknize/elasticsearch,gingerwizard/elasticsearch,gingerwizard/elasticsearch,scorpionvicky/elasticsearch,GlenRSmith/elasticsearch,robin13/elasticsearch,nknize/elasticsearch,uschindler/elasticsearch,scorpionvicky/elasticsearch,scorpionvicky/elasticsearch,scorpionvicky/elasticsearch,gingerwizard/elasticsearch,uschindler/elasticsearch,GlenRSmith/elasticsearch,HonzaKral/elasticsearch,HonzaKral/elasticsearch,HonzaKral/elasticsearch