fix string.isupper()

compiler/res/prog8lib/string.p8

@@ -399,15 +399,21 @@ fail    clc             ; yes, no match found, return with c=0
     }
 
     asmsub isupper(ubyte character @A) -> bool @Pc {
+        ; shifted petscii has 2 ranges that contain the upper case letters...
         %asm {{
-            cmp  #'A'
+            cmp  #97
             bcs  +
             rts
-+           cmp  #'Z'+1
-            bcc  +
++           cmp  #122+1
+            bcc  _yes
+            cmp  #193
+            bcs  +
+            rts
++           cmp  #218+1
+            bcc  _yes
             clc
             rts
-+           sec
+_yes        sec
             rts
         }}
     }

docs/source/libraries.rst

@@ -261,13 +261,16 @@ Provides string manipulation routines.
     Uppercases the PETSCII-string in place.
 
 ``lowerchar (char)``
-    Returns lowercased character.
+    Returns lowercased PETSCII character.
 
 ``upperchar (char)``
-    Returns uppercased character.
+    Returns uppercased PETSCII character.
 
-``isdigit (char)``, ``islower (char)``, ``isupper (char)``, ``isletter (char)``
-    Returns boolean if the character is a numerical digit 0-0, lowercase letter, uppercase letter, or any letter.
+``isdigit (char)``
+    Returns boolean if the character is a numerical digit 0-9
+
+``islower (char)``, ``isupper (char)``, ``isletter (char)``
+    Returns boolean if the character is a shifted-PETSCII lowercase letter, uppercase letter, or any letter.
 
 ``startswith (string, prefix) -> bool``
     Returns true if string starts with prefix, otherwise false