Add/update asset build filtering docs

[amdprophet]

Description

We've made a change to evaluate all asset builds. When there is more than one filtered asset build, the first asset build with the highest count of filters is selected for installation.

Implemented in PR sensu/sensu-go#4990.

Category

This is an issue with:

• errata (error in doc content)
• new doc (requires a completely new doc)
• update (add missing or refresh existing content)
• improvement (improve docs content or format)

Affected pages

https://docs.sensu.io/sensu-go/latest/plugins/assets/

Expected behavior

The new behaviour should be documented.

Current behavior

Currently the first filtered asset build is selected for installation. I'm not sure that this is documented so we may just need to add documentation for the new behaviour.