new documenation for struct pike_string

Rev: doc/internal/strings/add_shared_strings:1.1
Rev: doc/internal/strings/begin_shared_string:1.1
Rev: doc/internal/strings/binary_findstring:1.1
Rev: doc/internal/strings/end_shared_string:1.1
Rev: doc/internal/strings/findstring:1.1
Rev: doc/internal/strings/free_string:1.1
Rev: doc/internal/strings/make_shared_binary_string:1.1
Rev: doc/internal/strings/make_shared_string:1.1
Rev: doc/internal/strings/my_strcmp:1.1
Rev: doc/internal/strings/pike_string:1.1
Rev: doc/internal/strings/push_string:1.1
Rev: doc/internal/strings/string_replace:1.1