documentation about making use of custom vertex attributes

(cherry picked from commit 833a6b80123a830940f7b27682d74f93513a47c4)
9 jobs for custom_vertex_attrib_doc
Status Name Job ID Coverage
  Test
passed clang-c++11-Qt4 #3874
MultiThreads Linux

00:07:28

passed clang-c++11-Qt5 #3870
Linux MultiThreads

00:09:41

passed clang-c++98-Qt4 #3868
Linux MultiThreads

00:06:47

passed clang-c++98-Qt5 #3872
Linux MultiThreads

00:07:45

passed gcc-c++11-Qt4 #3865
Linux

00:03:21

passed gcc-c++11-Qt5 #3869
Linux

00:04:13

passed gcc-c++98-Qt4 #3867
Linux

00:02:39

passed gcc-c++98-Qt5 #3871
Linux

00:03:59

failed clang-c++11-Qt4 #3866
Linux MultiThreads

00:00:03