doc/improve-hint-text-doc (#156313)

This PR improve the Flutter documentation about the Textfield.hintText
property. On our e-commerce app, Samsung pass was not working and we
discovered that, after using hintText : Samsung pass autofill was
working perfectly.

Example (that can help) : 
```dart
 TextFormField(
   autofillHints: const <String>[
    // Order is important, username must be first
     AutofillHints.username,
     AutofillHints.email,
   ],
   keyboardType: TextInputType.emailAddress,
   decoration: InputDecoration(
     hintText: "What do you want, it doesn't matter but it's important to use it",
  ),
 ),
 ...
 TextFormField(
   autofillHints: const <String>[AutofillHints.password],    
   obscureText: true,
   keyboardType: TextInputType.visiblePassword,
   decoration: InputDecoration(
     hintText: "What do you want, it doesn't matter but it's important to use it",
   ),
 ),
```

## Pre-launch Checklist

- [X] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [X] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [X] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [X] I signed the [CLA].
- [X] I listed at least one issue that this PR fixes in the description
above.
- [X] I updated/added relevant documentation (doc comments with `///`).
- [X] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [X] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [X] All existing and new tests are passing.

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Co-authored-by: Enguerrand ARMINJON WINDOWS <[email protected]>

Author: EArminjon

packages/flutter/lib/src/widgets/editable_text.dart

@@ -1833,6 +1833,10 @@ class EditableText extends StatefulWidget {
   ///   Autofill service. Enable the autofill service of your choice, and make
   ///   sure there are available credentials associated with your app.
   ///
+  /// Specifying [InputDecoration.hintText] may also help autofill services
+  /// (like Samsung Pass) determine the expected content type of an input field,
+  /// although this is typically not required when autofillHints are present.
+  ///
   /// #### I called `TextInput.finishAutofillContext` but the autofill save
   /// prompt isn't showing
   ///