docs: add remaining docs for classes in app/logical.

app/logical/set_diff.rb

@@ -1,6 +1,12 @@
+# Calculate a diff between two sets of strings. Used to calculate diffs between
+# artist URLs and between other names. Tries to compare each set to detect which
+# strings were added, which were removed, and which were changed.
 class SetDiff
   attr_reader :additions, :removals, :added, :removed, :changed, :unchanged
 
+  # Initialize a diff between two sets of strings.
+  # @param this_list [Array<String>] the new list of strings
+  # @param other_list [Array<String>] the old list of strings
   def initialize(this_list, other_list)
     this, other = this_list.to_a, other_list.to_a
 
@@ -10,6 +16,12 @@ class SetDiff
     @added, @removed, @changed = changes(additions, removals)
   end
 
+  # Returns the strings that were either completely newly added, completely
+  # removed, or simply updated.
+  # @param added [Array<String>] the strings that were added to this_list
+  # @param removed [Array<String>] the strings that were removed from other_list
+  # @return [Array<(Array<String>, Array<String>, Array<String>)>] the list of
+  #   additions, removals, and changed strings.
   def changes(added, removed)
     changed = []
 
@@ -24,6 +36,9 @@ class SetDiff
     [added, removed, changed]
   end
 
+  # Finds the strings most similar to the `candidate` string among the `candidates` set.
+  # @param string [String] the string to find closest matches with in the `candidates` set.
+  # @param candidates [Array<String>] the set of candidate strings
   def find_similar(string, candidates, max_dissimilarity: 0.70)
     distance = ->(other) { ::DidYouMean::Levenshtein.distance(string, other) }
     max_distance = string.size * max_dissimilarity