glm includes are not explicit

[artificiel]

as per https://forum.openframeworks.cc/t/cant-find-glm-roundmultiple/41124 it's not evident for a fresh user to know to search for missing includes.

if including ofMain does not provide a functionality, it should be stated in the docs (in this case it does not https://openframeworks.cc/documentation/glm/gtc_round/#!show_glm::roundMultiple)

it's easy enough to make a change for that specific include in https://github.com/openframeworks/ofSite/blob/master/documentation/glm/gtc_round_functions.markdown, but it feels as if that comes from some automated generation from the glm source docs -- or does it actually need to be manually written?