minor symfony#2348 [Autocomplete][Doc] Mention preload can be set to false (MatTheCat)

This PR was merged into the 2.x branch.

Discussion
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[Autocomplete][Doc] Mention `preload` can be set to `false`

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| Issues        | N/A
| License       | MIT

Setting `preload` to `false` is useful when you only want to display search results; that way nothing is loaded on focus.

Commits
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4feaeb0 [Autocomplete][Doc] Mention `preload` can be set to `false`

Author: Kocal

src/Autocomplete/doc/index.rst

@@ -272,6 +272,7 @@ to the options above, you can also pass:
 ``preload`` (default: ``focus``)
     Set to ``focus`` to call the ``load`` function when control receives focus.
     Set to ``true`` to call the ``load`` upon control initialization (with an empty search).
+    Set to ``false`` not to call the ``load`` function when control receives focus.
 
 ``extra_options`` (default ``[]``)
     Allow you to pass extra options for Ajax-based autocomplete fields.