- tagging plaintext "mbuf", "mbuf cluster", and "mbuf chain"
with .Vt (variable type) since all of them are ways of managing
data, i.e., they can be seen as data types;
- using .Vt/.Va instead of .Li (literal) where appropriate;
- tagging plaintext words that actually refer to function arguments
with .Fa.
Suggested by: ru
7b2fd1831f