Update documentation of warnings options and strictness levels. The warning categories ‘cross’ and ‘portability-recursive’ were not mentioned in the manual. Also clarify the relationship between warnings categories and strictness levels, and streamline the description of strictness levels by merging the “Gnits” section into the “Strictness” section. * doc/automake.texi (Gnits, Strictness): Combine these sections. Minor revisions to explanation of strictness levels. (automake Invocation): Add documentation of all the warnings categories that have been added since the last time this section was updated. Minor clarifications.