improve labelalias description for negative numbers

Author: archmoj

src/plots/cartesian/layout_attributes.js

@@ -642,6 +642,8 @@ module.exports = {
             'For example using {US: \'USA\', CA: \'Canada\'} changes US to USA',
             'and CA to Canada. The labels we would have shown must match',
             'the keys exactly, after adding any tickprefix or ticksuffix.',
+            'For negative numbers the minus sign symbol used (U+2212) is wider than the regular ascii dash.',
+            'That means you need to use −1 instead of -1.',
             'labelalias can be used with any axis type, and both keys (if needed)',
             'and values (if desired) can include html-like tags or MathJax.'
         ].join(' ')