expanded explanation about self vs *self

exercises/047_methods.zig

@@ -37,6 +37,24 @@
 //    self, others use a lowercase version of the type name, but feel
 //    free to use whatever is most appropriate.
 //
+//    "But hold on," you say, eyeing a() and b() suspiciously, "why
+//    does one take 'self' and another take '*self'?" Sharp eye!
+//
+//    It all comes down to a single question: does the function need
+//    to CHANGE the struct?
+//
+//      * Needs to change it?  Take a pointer (*Bar). Without it you'd
+//        be scribbling on a COPY, and your changes would evaporate the
+//        instant the function returns. Poof.
+//      * Only reads it?  Plain Bar is just fine.
+//        (For a big, bulky struct you might still write *const Bar to
+//        avoid copying it around, but for small ones a copy is cheap.)
+//
+//    You'll see this below: zap() takes 'self: HeatRay' by value
+//    because it only reads the ray's damage, but it takes the alien
+//    as '*Alien' because zapping is supposed to HURT - and that means
+//    changing the alien's health for real, not on a throwaway copy.
+//
 // Okay, you're armed.
 //
 // Now, please zap the alien structs until they're all gone or