minor #10222 [ExpressionLanguage] added some documentation about functions (fabpot)

This PR was merged into the 2.4 branch.

Discussion
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[ExpressionLanguage] added some documentation about functions

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #10216
| License       | MIT
| Doc PR        | n/a

This adds documentation about how to define a function in the expression language.

Commits
-------

600d481 [ExpressionLanguage] added some documentation about functions

src/Symfony/Component/ExpressionLanguage/ExpressionLanguage.php

@@ -92,6 +92,18 @@ class ExpressionLanguage
     /**
      * Registers a function.
      *
+     * A function is defined by two PHP callables. The callables are used
+     * by the language to compile and/or evaluate the function.
+     *
+     * The first function is used at compilation time and must return a
+     * PHP representation of the function call (it receives the function
+     * arguments as arguments).
+     *
+     * The second function is used for expression evaluation and must return
+     * the value of the function call based on the values defined for the
+     * expression (it receives the values as a first argument and the function
+     * arguments as remaining arguments).
+     *
      * @param string   $name      The function name
      * @param callable $compiler  A callable able to compile the function
      * @param callable $evaluator A callable able to evaluate the function