Provide documentation for reactive Redis Template API [DATAREDIS-619]

**[Mark Paluch](https://jira.spring.io/secure/ViewProfile.jspa?name=mp911de)** opened **[DATAREDIS-619](https://jira.spring.io/browse/DATAREDIS-619?redirect=false)** and commented


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**Referenced from:** pull request https://github.com/spring-projects/spring-data-redis/pull/280

**Backported to:** [2.0.1 (Kay SR1)](https://github.com/spring-projects/spring-data-redis/milestone/85?closed=1)
