Introduce a constant that gives us the memory allocation overhead of
a serialization buffer and use that to specify "round" buffer sizes
(of 1kByte) instead of using seamingly magic sizes.

Once at it, fix a few typos in the doxygen strings.

* subversion/include/private/svn_temp_serializer.h
  (): include svn_string.h to make sure svn_stringbuf_t is defined
  (SVN_TEMP_SERIALIZER__OVERHEAD): define new constant
  (svn_temp_serializer__init,
   svn_temp_serializer__init_append,
   svn_temp_serializer__pop,
   svn_temp_serializer__revolve): fix typos in docstrings

* subversion/libsvn_fs_fs/dag.c
  (svn_fs_fs__dag_serialize): use new constant to calculate initial
   serialization buffer size 

* subversion/libsvn_fs_fs/temp_serializer.c
  (svn_fs_fs__serialize_node_revision): ditto

Suggested by: philip

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1448587 13f79535-47bb-0310-9956-ffa450edef68