Clarify usage comment for set() functions

[mr-j-tree]

It was not immediately clear that supplying a value of false for the "exists" argument has a special meaning. It causes the "NX" argument to be sent to Redis, as opposed to no argument when supplying a value of nil. At least that was my experience when using this function for the first time - the implications of true/false/nil were not explained until I read the source code for this function.

Description

Motivation and Context

How Has This Been Tested?

Checklist:

• [x] I have submitted a CLA form
• [x] If applicable, I have updated the documentation accordingly.
• [x] If applicable, I have added tests to cover my changes.

[CLAassistant]

All committers have signed the CLA.

[ianpartridge]

Thanks!