RFE: Comb flag warning

Currently if a PDF form text field has it's Comb flag set true but the max length hasn't been set then PDFBox silently ignores the field and doesn't render it.

It would be helpful for PDFBox to at least issue a warning that the field cannot be rendered as a Comb due to this.

It may be worth noting that if pdForm.setNeedAppearances is set true, and after setting fields pdForm.refreshAppearances() is invoked that Comb fields without max length specified do display in Acrobat and Edge. However without both of these conditions they also don't display, just like PDFBox currently.

Perhaps worth considering aligning PDFBox to behave in a similar fashion then, and to still issue the warning.