Add important to utilities, mention it's usage and global variable

Author: mdo

site/content/docs/5.0/utilities/api.md

@@ -109,11 +109,11 @@ $utilities: (
 Output:
 
 ```css
-.opacity-0-hover:hover { opacity: 0; }
-.opacity-25-hover:hover { opacity: .25; }
-.opacity-50-hover:hover { opacity: .5; }
-.opacity-75-hover:hover { opacity: .75; }
-.opacity-100-hover:hover { opacity: 1; }
+.opacity-0-hover:hover { opacity: 0 !important; }
+.opacity-25-hover:hover { opacity: .25 !important; }
+.opacity-50-hover:hover { opacity: .5 !important; }
+.opacity-75-hover:hover { opacity: .75 !important; }
+.opacity-100-hover:hover { opacity: 1 !important; }
 ```
 
 ### Responsive utilities
@@ -139,50 +139,50 @@ $utilities: (
 Output:
 
 ```css
-.opacity-0 { opacity: 0; }
-.opacity-25 { opacity: .25; }
-.opacity-50 { opacity: .5; }
-.opacity-75 { opacity: .75; }
-.opacity-100 { opacity: 1; }
+.opacity-0 { opacity: 0 !important; }
+.opacity-25 { opacity: .25 !important; }
+.opacity-50 { opacity: .5 !important; }
+.opacity-75 { opacity: .75 !important; }
+.opacity-100 { opacity: 1 !important; }
 
 @media (min-width: 576px) {
-  .opacity-sm-0 { opacity: 0; }
-  .opacity-sm-25 { opacity: .25; }
-  .opacity-sm-50 { opacity: .5; }
-  .opacity-sm-75 { opacity: .75; }
-  .opacity-sm-100 { opacity: 1; }
+  .opacity-sm-0 { opacity: 0 !important; }
+  .opacity-sm-25 { opacity: .25 !important; }
+  .opacity-sm-50 { opacity: .5 !important; }
+  .opacity-sm-75 { opacity: .75 !important; }
+  .opacity-sm-100 { opacity: 1 !important; }
 }
 
 @media (min-width: 768px) {
-  .opacity-md-0 { opacity: 0; }
-  .opacity-md-25 { opacity: .25; }
-  .opacity-md-50 { opacity: .5; }
-  .opacity-md-75 { opacity: .75; }
-  .opacity-md-100 { opacity: 1; }
+  .opacity-md-0 { opacity: 0 !important; }
+  .opacity-md-25 { opacity: .25 !important; }
+  .opacity-md-50 { opacity: .5 !important; }
+  .opacity-md-75 { opacity: .75 !important; }
+  .opacity-md-100 { opacity: 1 !important; }
 }
 
 @media (min-width: 992px) {
-  .opacity-lg-0 { opacity: 0; }
-  .opacity-lg-25 { opacity: .25; }
-  .opacity-lg-50 { opacity: .5; }
-  .opacity-lg-75 { opacity: .75; }
-  .opacity-lg-100 { opacity: 1; }
+  .opacity-lg-0 { opacity: 0 !important; }
+  .opacity-lg-25 { opacity: .25 !important; }
+  .opacity-lg-50 { opacity: .5 !important; }
+  .opacity-lg-75 { opacity: .75 !important; }
+  .opacity-lg-100 { opacity: 1 !important; }
 }
 
 @media (min-width: 1200px) {
-  .opacity-xl-0 { opacity: 0; }
-  .opacity-xl-25 { opacity: .25; }
-  .opacity-xl-50 { opacity: .5; }
-  .opacity-xl-75 { opacity: .75; }
-  .opacity-xl-100 { opacity: 1; }
+  .opacity-xl-0 { opacity: 0 !important; }
+  .opacity-xl-25 { opacity: .25 !important; }
+  .opacity-xl-50 { opacity: .5 !important; }
+  .opacity-xl-75 { opacity: .75 !important; }
+  .opacity-xl-100 { opacity: 1 !important; }
 }
 
 @media (min-width: 1400px) {
-  .opacity-xxl-0 { opacity: 0; }
-  .opacity-xxl-25 { opacity: .25; }
-  .opacity-xxl-50 { opacity: .5; }
-  .opacity-xxl-75 { opacity: .75; }
-  .opacity-xxl-100 { opacity: 1; }
+  .opacity-xxl-0 { opacity: 0 !important; }
+  .opacity-xxl-25 { opacity: .25 !important; }
+  .opacity-xxl-50 { opacity: .5 !important; }
+  .opacity-xxl-75 { opacity: .75 !important; }
+  .opacity-xxl-100 { opacity: 1 !important; }
 }
 ```
 
@@ -223,21 +223,25 @@ $utilities: (
 Output:
 
 ```css
-.opacity-0 { opacity: 0; }
-.opacity-25 { opacity: .25; }
-.opacity-50 { opacity: .5; }
-.opacity-75 { opacity: .75; }
-.opacity-100 { opacity: 1; }
+.opacity-0 { opacity: 0 !important; }
+.opacity-25 { opacity: .25 !important; }
+.opacity-50 { opacity: .5 !important; }
+.opacity-75 { opacity: .75 !important; }
+.opacity-100 { opacity: 1 !important; }
 
 @media print {
-  .opacity-print-0 { opacity: 0; }
-  .opacity-print-25 { opacity: .25; }
-  .opacity-print-50 { opacity: .5; }
-  .opacity-print-75 { opacity: .75; }
-  .opacity-print-100 { opacity: 1; }
+  .opacity-print-0 { opacity: 0 !important; }
+  .opacity-print-25 { opacity: .25 !important; }
+  .opacity-print-50 { opacity: .5 !important; }
+  .opacity-print-75 { opacity: .75 !important; }
+  .opacity-print-100 { opacity: 1 !important; }
 }
 ```
 
+## Importance
+
+All utilities generated by the API include `!important` to ensure they override components and modifier classes as intended. You can toggle this setting globally with the `$enable-important-utilities` variable (defaults to `true`).
+
 ## Using the API
 
 Now that you're familiar with how the utilities API works, learn how to add your own custom classes and modify our default utilities.