added the syntax for alpha() function

Author: dletorey

css/functions.json

@@ -15,6 +15,14 @@
     "status": "standard",
     "mdn_url": "https://developer.mozilla.org/docs/Web/CSS/acos"
   },
+  "alpha()": {
+    "syntax": "alpha( [from <color>] / [<alpha-value>] )",
+    "groups": [
+      "CSS Color"
+    ],
+    "status": "standard",
+    "mdn_url": "https://developer.mozilla.org/docs/Web/CSS/Reference/Values/color_value/alpha"
+  },
   "anchor()": {
     "syntax": "anchor( <anchor-name>? && <anchor-side>, <length-percentage>? )",
     "groups": [