missing --doc-base option

[Karl Berry]

A few days ago, gnulib-tool --import (or --update) started reporting 

gnulib-tool: *** missing --doc-base option
gnulib-tool: *** Stop.

The help msg says "doc" is supposed to be used as a default, but
apparently not.  I looked briefly at the script and surmise that the
default is only effective if the cache file doesn't exist, or something
like that.

So I just ran  gnulib-tool --doc-base doc  to get it into gnulib-cache.m4,
which is fine, but it seems the behavior or the error message or the
documentation could be improved.

Thanks,
karl