I think I’ve found a bug in the OpenGL ES 3.x manpages, at least as hosted on the web. They list the return type of glFramebufferRenderbuffer as GLsync, while the headers and ES 2.x manpages list it as void. The incorrect return type type appears on the OpenGL ES 3.0, 3.1 and 3.2 manpages.
I tried reporting this to manpages {at} opengldotorg but that mail server seems to be having issues (advertising TLS support but refusing TLS connections with “454 TLS currently unavailable”).