v.in.ascii: add second warning about import of files without category column

[jidanni]

On man v.in.ascii after

Import of files without category column If the input file does not contain a category column, there is the possibility to auto-generate these IDs (categories). To automatically add an additional column named 'cat', the cat parameter must be set to the virtual column number 0 (cat=0). This is the default action if the cat parameter is not set.

add an additional warning that these will be unique categories. In fact: 1, 2, 3...