Documented FormTypeGuesserInterface
Added references
Applied comments
feature #3597 Document how to create a custom type guesser (WouterJ)

This PR was merged into the 2.3 branch.

Discussion
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Document how to create a custom type guesser

| Q   | A
| --- | ---
| Doc fix? | no
| New docs? | yes
| Applies to | all
| Fixed tickets | #481

Commits
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0d37a3b Applied comments
3b3cd6f Added references
f36fdb7 Documented FormTypeGuesserInterface