RFR (S) 8241093: Shenandoah: editorial changes in flag descriptions

[Roman Kennke]

Looks good, thanks for doing it!

Roman

> RFE:
>   https://bugs.openjdk.java.net/browse/JDK-8241093
> 
> Webrev:
>   https://cr.openjdk.java.net/~shade/8241093/webrev.01/
> 
> This is a good time to polish any phrasing.
> 
> Testing: fastdebug build
>