[docs] Fix typos & formatting in filled-input (#13317)

* [docs] Fixes typos & formatting in filled-input

* [x]  I have followed (at least) the [PR section of the contributing guide](https://github.com/mui-org/material-ui/blob/master/CONTRIBUTING.md#submitting-a-pull-request).

Fixes a typo in filled-input documentation.

* let's merge

packages/material-ui/src/FilledInput/FilledInput.js

@@ -31,7 +31,7 @@ export const styles = theme => {
         backgroundColor: light ? 'rgba(0, 0, 0, 0.12)' : 'rgba(255, 255, 255, 0.12)',
       },
     },
-    /* Styles applied to the root element. */
+    /* Styles applied to the root element if `disableUnderline={false}`. */
     underline: {
       '&:after': {
         borderBottom: `2px solid ${theme.palette.primary[light ? 'dark' : 'light']}`,

pages/api/filled-input.md

@@ -56,7 +56,7 @@ This property accepts the following keys:
 | Name | Description |
 |:-----|:------------|
 | <span class="prop-name">root</span> | Styles applied to the root element.
-| <span class="prop-name">underline</span> | Styles applied to the root element.
+| <span class="prop-name">underline</span> | Styles applied to the root element if `disableUnderline={false}`.
 | <span class="prop-name">focused</span> | Styles applied to the root element if the component is focused.
 | <span class="prop-name">disabled</span> | Styles applied to the root element if `disabled={true}`.
 | <span class="prop-name">adornedStart</span> | Styles applied to the root element if `startAdornment` is provided.