bswap.3: bswap_*() are implemented using functions

bswap.3: bswap_*() are implemented using functions

See <bits/byteswap.h> in glibc.
These macros call functions of the form __bswap_N(),
which use uintN_t.

Even though it's true that they are macros,
it's transparent to the user.

The user will see their results casted to unsigned types
after the conversion due to the underlying functions,
so it's better to document these as the underlying functions,
specifying the types.

Signed-off-by: Alejandro Colomar <alx.manpages@gmail.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>