Alert users to not change collections in user callbacks while iterating

[gheber]

H5Iiterate is a case in point, but all modules should be reviewed.

[hyoklee]

https://docs.hdfgroup.org/hdf5/develop/group___h5_i_u_d.html#ga3bc2a11a594bf484cc5dc69ac987d0f4

[hyoklee]

Update H5Ipublic.h

[hyoklee]

@gheber , what do you mean by "collections" in this issue?

[gheber]

Mostly groups (= collections of links), attribute collections of objects, etc. I.e., don't change a group that you are iterating. OK?

[hyoklee]

I still don't understand. Please assign this to someone who wrote this function or is familiar with.