How do I tag a single wayside grave

Individual graves have received little mapping attention on OSM so far, except for graves of famous people, such as that of Jim Morrison which is tagged historic=grave. There is however a key cemetery which has been used to provide a context for micromapping of cemeteries, and cemetery=grave has 600 odd uses. Although your particular grave is not in a grave yard, I would use this tag.

Only use historic=grave if there is particular historic interest; and historic=memorial should be reserved for true memorials (although of course they may incorporate a grave or graves, such as ossuaries associated with battlefields: such as those on the Alpine front in Italy and at Donaumont). (If all graves used historic=memorial then I could not have used Nominatim to find my examples, whereas now one can ask for 'memorial near Verdun').

At least one OSM contributor has special knowledge in this area having done civil service in lieu of military service working in graveyards.

There is considerable scope within OSM for mapping cemeteries (particularly war cemeteries) as it is often difficult to find a specific grave when visiting a cemetery after a number of years. It is for this reason I have mapped this cemetery in some detail as I visit it perhaps every 10 years. Each time I have difficulty finding my uncle's grave and I have never seen that of my great-grandmother.