add explanation for rounding division

scripts/ci/gitlab/check-each-crate.sh

@@ -19,6 +19,14 @@ readarray -t workspace_crates < <(\
 )
 
 crates_total=${#workspace_crates[*]}
+
+# We add `crates_total % groups_total > 0` (which evaluates to 1 in case there's a remainder for
+# `crates_total % groups_total`) to round UP `crates_total / groups_total` 's potentially-fractional
+# result to the nearest integer. Doing that ensures that we'll not miss any crate in case
+# `crates_total / groups_total` would normally result in a fractional number, since in those cases
+# Bash would round DOWN the result to the nearest integer. For example, if `crates_total = 5` and
+# `groups_total = 2`, then `crates_total / groups_total` would round down to 2; since the loop below
+# would then step by 2, we'd miss the 5th crate.
 crates_per_group=$(( (crates_total / groups_total) + (crates_total % groups_total > 0) ))
 
 group=1