Expand documentation of declaration-block-no-redundant-longhand-prope… (

#3165)

lib/rules/declaration-block-no-redundant-longhand-properties/README.md

@@ -60,6 +60,10 @@ This rule complains when the following shorthand properties can be used:
 -   `text-emphasis`
 -   `mask`
 
+**Please note** that properties are considered to be redundant if they may be written shorthand according to the specification, **regardless of the behavior of any individual browser**. For example, due to Internet Explorer's implementation of Flexbox, [it may not be possible to use the shorthand property `flex`](https://github.com/philipwalton/flexbugs#flexbug-8), but the longhand form is still considered a violation. 
+
+Flexbox-related properties can be ignored using `ignoreShorthands: ["/flex-/"]` (see below).
+
 ## Options
 
 ### `true`