Update documentation of cache value types, following the memcache fixes
in r1570642 and r1585686 which involved a change from svn_string_t to
svn_stringbuf_t.

* subversion/include/private/svn_cache.h
  (svn_cache__create_inprocess): Add missing documentation.
  (svn_cache__create_memcache,
   svn_cache__create_membuffer_cache): Update doc strings.

* subversion/libsvn_fs_fs/fs.h
  (fs_fs_data_t): Update the documentation of 'fulltext_cache'.

* subversion/libsvn_fs_x/fs.h
  (fs_x_data_t): Same.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1586352 13f79535-47bb-0310-9956-ffa450edef68