Document pushenv support in modextravars

[Micket]

I happened to see in the code that we support "pushenv" for modextravars, but i can't find that we docuent it anywhere.

modextravars['TEST_PUSHENV'] = {'value': '123', 'pushenv': True}

Not that I have any need for it, i just closed an old issue asking for this, and i i found in the code that we actually already support it in secret.

https://docs.easybuild.io/version-specific/easyconfig-parameters/#modules-parameters
does not say much