update: update receiver type description. (#2926)
update: update receiver type description. (#2926)

* Update receiver type description.

There is no explicit information whether extensions can be provided on interfaces. 
This makes it clearer to the new reader.

* add receiver type info

Make it clear that extensions can be added on interface.
JetBrains/kotlin-web-site,JetBrains/kotlin-web-site,hltj/kotlin-web-site-cn,hltj/kotlin-web-site-cn,hltj/kotlin-web-site-cn,hltj/kotlin-web-site-cn,JetBrains/kotlin-web-site,hltj/kotlin-web-site-cn,JetBrains/kotlin-web-site,JetBrains/kotlin-web-site,JetBrains/kotlin-web-site,hltj/kotlin-web-site-cn