more documentation on internals added

Rev: doc/internal/low_level/hashmem:1.1
Rev: doc/internal/low_level/init_memsearch:1.1
Rev: doc/internal/low_level/low_level:1.1
Rev: doc/internal/low_level/memory_search:1.1
Rev: doc/internal/low_level/reorder:1.1
Rev: doc/internal/low_level/set_close_on_exec:1.1
Rev: doc/internal/low_level/set_nonblocking:1.1
Rev: doc/internal/low_level/set_read_callback:1.1
Rev: doc/internal/low_level/set_write_callback:1.1
Rev: doc/internal/low_level/xalloc:1.1
Rev: doc/internal/pike/SETJMP:1.1
Rev: doc/internal/pike/SET_ONERROR:1.1
Rev: doc/internal/pike/UNSETJMP:1.1
Rev: doc/internal/pike/UNSET_ONERROR:1.1
Rev: doc/internal/pike/error:1.2
Rev: doc/internal/pike/throw:1.1