Record why the RubyGems half keeps off

The two sides of one setting now disagree on `off`, so the list has to
say why it stays wider here. Nothing on the way in turns `off` into a
boolean, and nothing on this side shares the single vocabulary that
Bundler's boolean settings all run through.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: hsbt

lib/rubygems/config_file.rb

@@ -852,11 +852,18 @@ def warn_credential_store_fallback
     alert_warning "Could not write the API key to the credential store, so it was written to #{credentials_path} in plain text."
   end
 
-  # Anything that reads as a boolean is one, so RUBYGEMS_CREDENTIAL_STORE=0
-  # turns the store off rather than naming a backend gem "0".
   CREDENTIAL_STORE_OFF = %w[false 0 no off f n].freeze
   CREDENTIAL_STORE_ON = %w[true 1 yes on t y].freeze
 
+  #--
+  # Anything that reads as a boolean is one, so RUBYGEMS_CREDENTIAL_STORE=0
+  # turns the store off rather than naming a backend gem "0". Both lists are
+  # this setting's own. Nothing on the way here turns `off` into a boolean, so
+  # a gemrc saying `off` arrives as a String just as the environment variable
+  # does. Bundler leaves `off` out of its half because every one of its
+  # boolean settings shares a single vocabulary that has never had it. Nothing
+  # here shares that constraint.
+
   def normalize_credential_store(value, default)
     # An environment variable can carry bytes String#downcase would reject.
     normalized = value.to_s.b.downcase