svnadmin dump /svn/old_repos > ./repository.dump
svndumpfilter include path/to/docs path/to/anotherdir --drop-empty-revs --renumber-revs --preserve-revprops < ./repository.dump > ./docs_only.dump
svnadmin load /svn/new_repos < ./docs_only.dump

Courtesy of stackoverflow.

Note that the switches –drop-empty-revs –renumber-revs will change revision numbers. Omit these switches if preserving version numbers is important to you.