man: shorten list of partition types a bit

Tables with only one column aren't really tables, they are lists. And if
each cell only consists of a single word, they are probably better
written in a single line. Hence, shorten the man page a bit, and list
boot loader spec partition types in a simple sentence.

Also, drop "root-secondary" from the list. When dissecting images we'll
upgrade "root-secondary" to "root" if we mount it, and do so only if
"root" doesn't exist. Hence never mention "root-secondary" as we never
will mount a partition under that id.