minor #20630 [HttpFoundation] Add links to RFC-7231 (nicolas-grekas)

This PR was merged into the 2.7 branch.

Discussion
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[HttpFoundation] Add links to RFC-7231

| Q             | A
| ------------- | ---
| Branch?       | 2.7
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | no
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

ping @xabbuh

Commits
-------

c17a85b [HttpFoundation] Add links to RFC-7231

src/Symfony/Component/HttpFoundation/Request.php

@@ -1466,6 +1466,8 @@ class Request
     /**
      * Checks whether the method is safe or not.
      *
+     * @see https://tools.ietf.org/html/rfc7231#section-4.2.1
+     *
      * @param bool $andCacheable Adds the additional condition that the method should be cacheable. True by default.
      *
      * @return bool
@@ -1478,6 +1480,8 @@ class Request
     /**
      * Checks whether the method is cacheable or not.
      *
+     * @see https://tools.ietf.org/html/rfc7231#section-4.2.3
+     *
      * @return bool
      */
     public function isMethodCacheable()