[DOC] Document thread-safety of Digest algorithms

[adam12]

Using constants under the Digest:: namespace are loaded through const_missing, but this has been observed to not be thread-safe and can cause errors regarding Digest::Base. We should more prominently encourage the use of Digest() when loading dynamically or using an explicit require up front when the algorithm is known.

[rickhull]

I was involved in discussion around this on the Ruby discord. This looks like a clear improvement to me.