To stay consistent with the work started in 54627f991 ("DOC: config: add
context hint for proxy keywords") and 3d4e1e682 ("DOC: config: add context
hint for server keywords"), we add missing context hint for "guid" (both
proxy and server) keyword and "hash-key" server keyword that were added
during 3.0 development.
This may be backported in 3.0.