[Rd] is.vector could handle AsIs class better

[Jan Gorecki]

Dear R-devel,

AsIs class seems to be well handled by `typeof` and `mode` function.
Those two functions are being referred when explaining `is.vector`
behaviour in manual. Yet `is.vector` does not seem to be handling AsIs
class the same way.

is.vector(1L)
#[1] TRUE
is.vector(I(1L))
#[1] FALSE

Is there any reason behind this behaviour?
Could we have it supported so AsIs class is ignored when `is.vector`
is doing its job?

Best Regards,
Jan Gorecki