DQL custom functions: document TypedExpression

[janedbal]

Partially related to #11537

[SenseException]

👍 Please squash your 4 commits into 1.

[janedbal]

No problem. Just out of curiosity, why do you force contributors to squash it manually when you can do it with a single button?

[greg0ire]

For me, there are at least 2 reasons:

• Pressing that button forces us to come up with a commit message summarizing all the commits. Crafting that commit message should IMO be your job, since it's your changes.
• The commit hash stays the same before and after merging, when no squashing is involved. That can be handy when trying to figure out what branch contains a particular commit or whatnot.

[greg0ire]

Thanks @janedbal !