Allow alternate event/hook prototype declarations

The alternates must be some subset of the canonical prototype (the one
that's first declared) and allows users to define handlers for any
such prototype.  Example:

    # Prototype declarations
    global my_event: event(s: string, c: count);
    global my_event: event(c: count);
    global my_event: event();

    # Handler definitions
    event my_event(s: string, c: count) { print s, c; }
    event my_event(c: count) { print c; }
    event my_event() { }

This allows handlers to consume a subset of the arguments or even
re-order them.  This makes it easier to either extend an existing
event/hook's arguments and/or deprecate usages of certain prototypes.

src/ID.cc

@@ -431,8 +431,42 @@ void ID::DescribeReST(ODesc* d, bool roles_only) const
 			d->Add("`");
 			}
 		else
+			{
 			type->DescribeReST(d, roles_only);
 
+			if ( IsFunc(type->Tag()) )
+				{
+				auto ft = type->AsFuncType();
+
+				if ( ft->Flavor() == FUNC_FLAVOR_EVENT ||
+				     ft->Flavor() == FUNC_FLAVOR_HOOK )
+					{
+					const auto& protos = ft->Prototypes();
+
+					if ( protos.size() > 1 )
+						{
+						auto first = true;
+
+						for ( const auto& proto : protos )
+							{
+							if ( first )
+								{
+								first = false;
+								continue;
+								}
+
+							d->NL();
+							d->Add(":Type: :zeek:type:`");
+							d->Add(ft->FlavorString());
+							d->Add("` (");
+							proto.args->DescribeFieldsReST(d, true);
+							d->Add(")");
+							}
+						}
+					}
+				}
+			}
+
 		d->NL();
 		}