Fix bottom limit of exponent and add comments for the new "SI extended" feature

Author: dberardi99

src/plots/cartesian/axes.js

@@ -2070,7 +2070,7 @@ function beyondSI(exponent) {
 }
 
 function beyondSIExtended(exponent) {
-    return exponent > 32 || exponent < -32;
+    return exponent > 32 || exponent < -30;
 }
 
 function numFormat(v, ax, fmtoverride, hover) {

src/plots/cartesian/layout_attributes.js

@@ -908,7 +908,12 @@ module.exports = {
             'If *E*, 1E+9.',
             'If *power*, 1x10^9 (with 9 in a super script).',
             'If *SI*, 1G.',
-            'If *B*, 1B.'
+            'If *B*, 1B.',
+            
+            '*SI* uses prefixes from "femto" f (10^-15) to "tera" T (10^12).',
+            '*SI extended* covers instead the full SI range from "quecto" q (10^-30) to "quetta" Q (10^30).',
+            'If *SI* or *SI extended* is used and the exponent is beyond the above ranges, the formatting rule',
+            'will automatically be switched to the power notation.'
         ].join(' ')
     },
     minexponent: {