perlio.c: Add comments

The code is using '#ifdef EBCDIC' when it has nothing to do with EBCDIC;
but is a proxy for how things work on IBM boxes that use EBCDIC.  Note
this in the comments.

perlio.c

@@ -1991,7 +1991,9 @@ PerlIOBase_pushed(pTHX_ PerlIO *f, const char *mode, SV *arg, PerlIO_funcs *tab)
             SETERRNO(EINVAL, LIB_INVARG);
             return -1;
         }
-#ifdef EBCDIC
+#ifdef EBCDIC   /* Probably should refer to the affected OS's; the
+                   character set is just a proxy for what these platforms have
+                   in common */
         {
         /* The mode variable contains one positional parameter followed by
          * optional keyword parameters.  The positional parameters must be
@@ -2943,7 +2945,7 @@ PerlIO_importFILE(FILE *stdio, const char *mode)
 {
     dTHX;
     PerlIO *f = NULL;
-#ifdef EBCDIC
+#ifdef EBCDIC   /* Again, the character set is just a proxy for IBM-ese */
          int rc;
          char filename[FILENAME_MAX];
          fldata_t fileinfo;
@@ -3000,7 +3002,7 @@ PerlIO_importFILE(FILE *stdio, const char *mode)
                 PerlIOUnix_refcnt_inc(fd0);
                 setfd_cloexec_or_inhexec_by_sysfdness(fd0);
             }
-#ifdef EBCDIC
+#ifdef EBCDIC   /* Again, the character set is just a proxy for IBM-ese */
                 else{
                         rc = fldata(stdio,filename,&fileinfo);
                         if(rc != 0){