Explain pure behavior of `isolate` more explicitly
Explain pure behavior of `isolate` more explicitly

It wasn't immediately clear that calling `isolate` with both arguments
is a pure function.
Making this distinction explicit in the documentation should help others
understand how `isolate` behaves in both situations.

staltz/cycle,maskinoshita/cyclejs,usm4n/cyclejs,feliciousx-open-source/cyclejs,feliciousx-open-source/cyclejs,cyclejs/cyclejs,cyclejs/cyclejs,maskinoshita/cyclejs,usm4n/cyclejs,maskinoshita/cyclejs,feliciousx-open-source/cyclejs,feliciousx-open-source/cyclejs,maskinoshita/cyclejs,ntilwalli/cyclejs,cyclejs/cycle-core,ntilwalli/cyclejs,cyclejs/cyclejs,usm4n/cyclejs,ntilwalli/cyclejs,cyclejs/cyclejs,usm4n/cyclejs,staltz/cycle,ntilwalli/cyclejs,cyclejs/cycle-core