Prefer "inclusion" argument to "in" 🐹

[sunny]

Following #82, the name inclusion is clearer than in for input arguments.

Deprecation-wise, the idea is to:

• in this patch, soft-deprecate the in argument by allowing both syntaxes and removing the previous syntax from the documentation,
• later, deprecate the in argument with a warning on use,
• later, drop the in argument, on a major version upgrade.