[doc] Improve documentation for common_ifs
[doc] Improve documentation for common_ifs

This improves the English a bit and also restructures the list to
group similar functions/tasks together, adding some long-form text
about what's going on.

Signed-off-by: Rupert Swarbrick <290093e2ad2515ed2f14d749293c8cf8e02ca274@lowrisc.org>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan