Clarify int* and nat* generator documentation

[jmid]

On Discuss it was pointed out that the int_pos* documentation wasn't particularly well worded:

Anyway, I don’t advocate any change in the code, but maybe the documentation Gen (qcheck-core.QCheck2.Gen) can be made a bit more consistent (e.g., the doc of int_pos_small says “positive integers (0 included)”, the doc of int_pos says “non-strictly positive”).

https://discuss.ocaml.org/t/ann-qcheck-0-90-the-great-renaming/17613/5

This PR thus intends to better this by

• being more consistent about starting with Generates ...
• eliminating the slightly unusual non-strictly positive
• stating [0] inclusive consistently

On purpose I haven't touched the stated upper bounds (< 100, < [10_000]) as the intention is to revise some of these distributions anyway #368.