some documentation fixes

Rev: doc/internal/low_level/low_level:1.2
Rev: doc/internal/pike/SETJMP:1.2
Rev: doc/internal/pike/SET_ONERROR:1.2
Rev: doc/internal/pike/UNSETJMP:1.2
Rev: doc/internal/pike/UNSET_ONERROR:1.2
Rev: doc/internal/pike/data_types:1.1
Rev: doc/internal/pike/error:1.3
Rev: doc/internal/pike/error_handling:1.1
Rev: doc/internal/pike/fatal:1.2
Rev: doc/internal/pike/frame:1.2
Rev: doc/internal/pike/throw:1.2
Rev: doc/internal/pike/type_field:1.2
Rev: doc/internal/pike/types:1.2