Add suggestion for acronym-compatible "master/slave" replacements

Offer replacement terms for "master/slave" that share the same initials,
so that, eg. SPI buses can reuse MISO/MOSI.

Suggestions are "manager/subordinate" and "main/subsystem", with the
latter only suggested in singular form ("mains" is something else as a
noun).

From https://twitter.com/ktemkin/status/1271605835303489537 and
https://twitter.com/profhummel/status/1271899937647652870