minor symfony#7139 Add a warning about routes parameters names limitations (fancyweb, javiereguiluz)

This PR was merged into the 2.7 branch.

Discussion
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Add a warning about routes parameters names limitations

@wouterj @xabbuh

I'm sorry guys I had to do a new branch and a new PR because I failed in the old one. I shouldn't have used the fast GitHub edit method.

I fixed the indentation to 4 spaces.

Also I thought this kind of information fitted more in the "advanced" area along with the special parameters than in the basic examples.

Follow up to symfony#7133

Commits
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cadef60 Use "route placeholder" instead of "route parameter"
6b0f53c Add a warning about routes parameters names limitations

Author: xabbuh

routing.rst

@@ -466,6 +466,10 @@ a slash. URLs matching this route might look like:
     Symfony provides you with a way to do this by leveraging service container
     parameters. Read more about this in ":doc:`/routing/service_container_parameters`".
 
+.. caution::
+
+    A route placeholder name cannot start with a digit and cannot be longer than 32 characters.
+
 Special Routing Parameters
 ~~~~~~~~~~~~~~~~~~~~~~~~~~