[ExpressionLanguage] added some documentation about functions
[ExpressionLanguage] added some tests for the built-in constant() function
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
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600d481 [ExpressionLanguage] added some documentation about functions
Merge branch '2.4'

* 2.4:
  fixed various inconsistencies
  reduced recursion when building DumperPrefixCollection
  renamed variables - making next change more readable
  removing dead code.
  [ExpressionLanguage] added some tests for the built-in constant() function
  [ExpressionLanguage] added some documentation about functions
  [DomCrawler] Fixed filterXPath() chaining
  [DomCrawler] Fixed incorrect handling of image inputs