minor corrections to javadocs for new scan/reduce variants (#1835)
make javadoc for zip() variants more precise (#1858)
make javadocs for scan() more precise (#1857)
make javadocs for reduce() more precise (#1856)
clarify return value of publish() in javadocs (#1855)
clean up onBackpressureDrop() javadocs and make similar to those for onBackpressureBuffer() (#1854)
Improve asObservable() javadoc description (#1853)
remove links to .NET docs from our javadocs (#1852)