Make it more clear that the Desktop C++ requires isEarlyStartup: true

[badboy]

I have recently run into the issue that I used both the JS and the C++ api on desktop to fetch a new experiment variable.
The code disagreed.
It took me a while to understand that the C++ API only queries preferences and those preferences are only set for features with isEarlyStartup: true.

Later then I found https://experimenter.info/early-startup/what-do-it-do/ and https://experimenter.info/early-startup/how-early/#early-on-desktop
However nowhere it's clear that C++ always requires isEarlyStartup, even if it is queried later.

This should be made more clear in the docs.