docs(button): add explanation for FAB abbreviation (#5790)

Author: donroyco

src/lib/button/button.md

@@ -21,8 +21,9 @@ There are five button variants, each applied as an attribute:
 
 ### Theming
 Buttons can be colored in terms of the current theme using the `color` property to set the
-background color to `primary`, `accent`, or `warn`. By default, only FABs are colored; the default
-background color for `md-button` and `md-raised-button` matches the theme's background color.
+background color to `primary`, `accent`, or `warn`. By default, only FABs (Floating Action Button) 
+are colored; the default background color for `md-button` and `md-raised-button` matches the theme's 
+background color.
 
 ### Capitalization
 According to the Material design spec button text has to be capitalized, however we have opted not

src/lib/button/button.scss

@@ -12,7 +12,7 @@
   }
 }
 
-// Only flat buttons (not raised, fabs or icon buttons) have a hover style.
+// Only flat buttons (not raised, FABs or icon buttons) have a hover style.
 // Use the same visual treatment for hover as for focus.
 .mat-button:hover .mat-button-focus-overlay {
   opacity: 1;

src/lib/sidenav/sidenav.md

@@ -68,5 +68,5 @@ html, body, material-app, md-sidenav-container, .my-content {
 ```
 
 ### FABs inside sidenav
-For a sidenav with a FAB (or other floating element), the recommended approach is to place the FAB
+For a sidenav with a FAB (Floating Action Button) or other floating element, the recommended approach is to place the FAB
 outside of the scrollable region and absolutely position it.