documentation about making use of custom vertex attributes

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

00:07:28

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

00:09:41

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

00:06:47

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

00:07:45

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

00:03:21

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

00:04:13

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

00:02:39

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

00:03:59

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

00:00:03