bitbake-layers/create: Improve help message for layerid argument

bitbake-layers/create: Improve help message for layerid argument

'Layer ID' isn't a commonly used term in our help messages or
documentation, so clarify that this is the identifier used for the new
layer in BBFILE_COLLECTIONS.

Also clarify that the default is the basename of the layerdir argument
if a layer ID is not separately provided.

Signed-off-by: Paul Barker <paul@pbarker.dev>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>