Update instructions for generate() in usage.md

[mattstibbs]

Add additional guidance around generation of live numbers vs synthetic numbers.

[andylaw]

Thanks @mattstibbs - this is good.

There's a typo in the word "synthetic" on line 79, and can I suggest that the warning probably needs to be (a) more stark and (b) reference the potential for using LIVE numbers rather than synthetic ones. Something like:

!!! WARNING: Careless use of numbers generated from this library could result in data loss,
damage or compromise. Under no circumstances should these be used in queries against
live health service systems.

Would you be happy with that phrasing?

[mattstibbs]

Thanks for spotting the typo.

Agree we could make warning more stark, but I think it needs to be more explicit about how the risk can occur. I'll submit some revised phrasing.

[pacharanero]

Any update on this draft PR @mattstibbs? Note that documentation has had other updates in the interim period so please check your updates are still playing well with the current code.