chore: set expectation for function compatibility

chore: set expectation for function compatibility

Set clear boundaries for which functions are maintained for
compatibility with existing dracut modules.

Document the compatibility differences between dracut.sh and
dracut-functions.sh, and move any functions from dracut.sh to
dracut-functions.sh that are clearly required for dracut
module compatibility.

This commit moves all functions from dracut.sh to dracut-functions.sh
that are already documented in man/dracut.modules.7.adoc.