feature #7970 Dom crawler form examples (uikolas)

This PR was submitted for the master branch but it was merged into the 2.7 branch instead (closes #7970).

Discussion
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Dom crawler form examples

Added more examples, how to get crawler form.
I think for new users it's kinda useful, these examples.
Especially if form button doens't contains name

Commits
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36e3508 Small mistype edits
16db597 Changed text how to get button label
096c1c0 Small misspelling fixes
cdf1a5b Small typo fix
fce7657 Edited to show more ways, to get crawler form

Author: weaverryan

components/dom_crawler.rst

@@ -338,10 +338,19 @@ given text. This method is especially useful because you can use it to return
 a :class:`Symfony\\Component\\DomCrawler\\Form` object that represents the
 form that the button lives in::
 
-    $form = $crawler->selectButton('validate')->form();
+    // button example: <button id="my-super-button" type="submit">My super button</button>
+    
+    // you can get button by its label
+    $form = $crawler->selectButton('My super button')->form();
+    
+    // or by button id (#my-super-button) if the button doesn't have a label
+    $form = $crawler->selectButton('my-super-button')->form();
+    
+    // or you can filter the whole form, for example a form has a class attribute: <form class="form-vertical" method="POST">
+    $crawler->filter('.form-vertical')->form();
 
     // or "fill" the form fields with data
-    $form = $crawler->selectButton('validate')->form(array(
+    $form = $crawler->selectButton('my-super-button')->form(array(
         'name' => 'Ryan',
     ));