* include/abg-fwd.h: Correct doc-comment reference to
enum_type_decl.
* src/abg-comp-filter.cc: Fix doc-comment syntax.
* src/abg-comparison.cc (operator<<): In the diff_category
overload, fix code indentation.
* src/abg-default-reporter.cc (report): In the
class_or_union_diff overload, adjust comment to reflect that
the code is reporting changes between declaration-only and
defined types, in either direction.