div64: Correct inline documentation for `do_div'

Correct inline documentation for `do_div', which is a function-like
macro the `n' parameter of which has the semantics of a C++ reference:
it is both read and written in the context of the caller without an
explicit dereference such as with a pointer.

In the C programming language it has no equivalent for proper functions,
in terms of which the documentation expresses the semantics of `do_div',
but substituting a pointer in documentation is misleading, and using the
C++ notation should at least raise the reader's attention and encourage
to seek explanation even if the C++ semantics is not readily understood.

While at it observe that "semantics" is an uncountable noun, so refer to
it with a singular rather than plural verb.

Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>