docs: add documentation for various classes in app/logical.

app/logical/note_sanitizer.rb

@@ -1,3 +1,5 @@
+# Sanitizes the HTML used in notes. Only safe HTML tags, HTML attributes, and
+# CSS properties are allowed.
 module NoteSanitizer
   ALLOWED_ELEMENTS = %w[
     code center tn h1 h2 h3 h4 h5 h6 a span div blockquote br p ul li ol em
@@ -50,6 +52,9 @@ module NoteSanitizer
     vertical-align
   ]
 
+  # Sanitize a string of HTML.
+  # @param text [String] the HTML to sanitize
+  # @return [String] the sanitized HTML
   def self.sanitize(text)
     text.gsub!(/<( |-|3|:|>|\Z)/, "&lt;\\1")
 
@@ -76,6 +81,8 @@ module NoteSanitizer
     )
   end
 
+  # Convert absolute Danbooru links inside notes to relative links.
+  # https://danbooru.donmai.us/posts/1 is converted to /posts/1.
   def self.relativize_links(node:, **env)
     return unless node.name == "a" && node["href"].present?