Search instruction after adding custom properties to APIs is not clear

[shamin2021]

Description:
As mentioned in the API-M Docs custom properties can be added to an API and then can be searched in the search bar using the format
<property_name>:<property_value>

But as per the comment mentioned in the issue
When devportal visibility is enabled for a property when searching from Publisher, we need to search it in the following manner.
<property_name>__display:<property_value>

This is not clearly mentioned in the documentation.

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[Tharanidk]

This information is already available in the document.

Fixed with #8240