Thanks to the link to "http"https://mathematica.stackexchange.com/q/19902/14303" in jkuczm'sjkuczm's comment, I found a partial answer with Language`ExtendedDefinition
. For instance:
Language`ExtendedDefinition[Log]
Language`DefinitionList[Log->{ OwnValues->{}, SubValues->{}, UpValues->{}, DownValues->{}, NValues->{}, FormatValues->{HoldPattern[MakeBoxes[Log[BoxForm`b_,BoxForm`z_],TraditionalForm]]:>RowBox[{SubscriptBox[log,MakeBoxes[BoxForm`b,TraditionalForm]],(,MakeBoxes[BoxForm`z,TraditionalForm],)}]}, DefaultValues->{}, Messages->{HoldPattern[Log::usage]->Log[z] gives the natural logarithm of z (logarithm to base e). Log[b,z] gives the logarithm to base b. }, Attributes->{Listable,NumericFunction,Protected}}]
I say "partial" only because there is a slight mystery: in my notebook, I had to actually evaluate the symbol Log
before its Messages
attribute showed any value. Small point.