src: nitunit and docdown use the fence-tag to skip non-nit code

Signed-off-by: Jean Privat <jean@pryen.org>
nitunit: enforce the testing of docunits (unless explicitely fence-tagged)

Signed-off-by: Jean Privat <jean@pryen.org>
man: explain docunits in man nitdoc

Signed-off-by: Jean Privat <jean@pryen.org>
nitdoc: increase the verbosity level for the numerous skip messages

Signed-off-by: Jean Privat <jean@pryen.org>
tests: test tagged fences for test_docdown and nitunit

Signed-off-by: Jean Privat <jean@pryen.org>
all: update code to add nitish-fences in documentation

Signed-off-by: Jean Privat <jean@pryen.org>