[Chapel Merge] Add comments for why we disable merging/splitting

Branch: refs/heads/main
Revision: f346da7
Author: ronawho
Link: Unavailable
Log Message:

Merge pull request #19171 from ronawho/comment-je-merge-split-ugni

Add comments for why we disable merging/splitting under ugni

[reviewed by @gbtitus]

In #18738 we disabled merge/split hooks under ugni as a workaround to a
bug we didn't fully understand. We have since figured out that the
problem was that we had a single blob of memory spanning multiple memory
registration regions created by merging chunks, and this isn't something
ugni supports. #19127 added checks for this condition and this just
adds comments to indicate why we disable merge/split for ugni.

Modified Files:
M runtime/src/mem/jemalloc/mem-jemalloc.c

Compare: https://github.com/chapel-lang/chapel/compare/4e0bc2e60ded...f346da7e720c